"Dude, are you hungry? I haven't eaten since later this afternoon."
It is often held that time travel is difficult to talk about because of shortfalls in the English language. In his science fiction book The Restaurant At The End Of The Universe, Douglas Adams introduces a fictional author "Dr. Dan Streetmentioner" whose own Time-Traveller's Handbook Of 1001 Tense Formations explains at great and convoluted length how to describe events in your past but another person's future, events averted through time travel, events which you are, while speaking, travelling through time to avoid, and many more. This involves the introduction of many new verb tenses and convoluted verb conjugation rules for these tenses.
In fact, existing grammatical rules are already perfectly well-suited to talking about time travel. All we need to do is obey some conventions.
There are multiple distinct models of time travel which a universe may obey. In the simplest model, time travel is impossible and this entire discussion is moot. For the purposes of this first section, we shall assume the second-simplest model of time travel: that there is a single, perfectly internally consistent, rigid timeline, whose history cannot be altered, even though time travel is, to whatever degree, possible. Fictional universes obeying this model include The Hitchhiker's Guide To The Galaxy, Twelve Monkeys and Bill & Ted's Excellent Adventure.
The first step to time travel conversations is to abandon an assumption: objectivity.
When time travel doesn't exist, the past, present and future are (barring relativistic effects) absolute and universal. My past is your past is the world's past. But in a universe with time travel, you no longer necessarily share a future or a past with the person or people with whom you are speaking. Only the universe itself has an objective past and future anymore. Events in your past may actually be in the future chronologically, because you travelled back in time to where you are now. You may have experienced the same event from different angles at different times in your personal history.
Time is subjective, which means statements about "the past", "the present" and "the future" are ambiguous.
There's actually a lot of good news in this area, though. Firstly, space is already subjective. While we're facing each other, my left is your right, and if we're on different continents, then what's a mile away for me is ten thousand miles away for you. Conventions already exist to handle problems like this, and these conventions are very readily adapted for analogous situations where, for example, an event in my past is an event in your future, or when an event twenty years in your past is only twenty minutes in my past.
Secondly, the problem of subjective time already exists in the real world even without time travel. It's called the postal service (or more generally, asynchronous communication). Suppose an event is due to occur tomorrow, and you write a letter about it and send it standard class air mail to the other side of the world. By the time the recipient reads the letter, the event will be in the past. What tense should you use, then, when writing? Well, you would start off with "By the time you get this, the event described will be in the past!" and go on to explain the rest using "should" and "will have" and so on. It's not a big deal. Just pay attention and you can handle this.
None of this involves the introduction of exotic new verb tenses. All you need to do is drop cues and provide context to others in order to clarify your statements about events, while continuing to use existing grammar.
The second step after abandoning objectivity is to make sure that your conversation partner does the same. To put it another way, the first cue you should drop is to tell the other person that you're a time traveller. Otherwise, the rest of this is pointless.
If you wish to keep the existence of time travel a secret, then abandon these rules, forget everything you know about the future and consult a conventional English grammar handbook. Better still, avoid conversations with non-time-travellers entirely.
"The Greenwich Meridian lies at longitude 0 degrees 0 minutes 0 seconds" is always true no matter where in space you are standing, how fast you are travelling or in what direction.
Likewise, in a fixed history, events remain events no matter the perspective from which they are considered. All of history, in fact, can be thought of as a single static event. Thus, a fixed event in history, such as the President of Malawi being hit in the face with a custard pie on 13 August 1990, can always be correctly referred to using the present tense. "In 1990, the Malawian President is pied in the face" is correct no matter where in history you are when you say this - 1991, 1957, 1990 watching the event happen, travelling through time past 1990 in either direction. It also doesn't matter whether you're talking to somebody in person, talking to them in the past from the future, addressing a group of people at different eras, or writing a letter to future recipients unknown.
Assuming that you are speaking to somebody at the same location in history as you, and you aren't in the process of travelling past 1990 in a time machine, it is usually permissible to speak about events in the same general terms as you use in time-travel-free reality. In 1989: "The Malawian President will be pied in 1990". In 1991: "The Malawian President was pied in 1990". Also: "The Malawian President will be pied next year/tomorrow/any day now/soon" and "The Malawian President was pied recently/last year/yesterday/just now". Or omit the qualification entirely, as in "The Malawian President will be pied" and "The Malawian President was pied". In all of these cases, your conversation partner will (or should) assume from context that you are describing the relative locations in time of (1) the pieing and (2) the conversation with respect to objective history in general, not necessarily with respect to you or your conversation partner, both of whom may be time travellers with entirely more complicated perspectives on 1990.
The convention is easily breakable to confusing effect. In 2008: "The Malawian President will be pied in 1990". In 1957: "The Malawian President was pied in 1990". Both of these statements may be subjectively true from your perspective, if you are about to travel back in time to witness the pieing/just travelled back in time from witnessing the pieing. But it's very bad practice!
All of the above only holds if the pieing of the Malawian President is an "objective event" with no specific personal significance to you or your conversation partner, at which neither of you were present, and for which neither of you were responsible. Almost all historical events and news stories fall into this category. But when this is not the case, the rules become more complex:
In 1990, you pie the Malawian President in the face and then travel back to 1983.
You already know it's acceptable to use the present tense, "I pie the Malawian President in 1990", although this is a somewhat odd phrasing in this case.
But you must be careful using phrases like "I will pie a Malawian in 1990" or just "I'm going to custard-pie the President of Malawi". It is clearer to say "I pied that guy", using the past tense.
Why? Because you're talking about yourself now, not the world at large. This event is part of your subjective past and always will be, no matter where in time you travel. When speaking about events directly affecting yourself, it will usually be assumed by your conversation partner that you are speaking from your own subjective perspective, because this is the most normal way to refer to events in your own timeline. "I will assault the Malawian head of state using custard" implies that this is a plan you intend to carry out sometime in your subjective future, which it isn't. A passive voice, "The Malawian President will be pied by me" is clearer on this point, because it more heavily emphasises the object of the statement.
Similar rules apply when referring to other people. In 1990, let's say your mother pies the President instead. Then you travel back to 1983 to meet her. This time, "I watched you pie the President of Malawi" is appropriate, but "You pied the President of Malawi" is not because this event is not her past as of 1983. "You will pie him" is better.
If, in 1990, your mother threw a custard pie in your face, and you go back to 1983 to confront her about it, both sets of rules apply. "You pied me" and "You're going to pie me" are equally true and meaningful. Take care to ensure that it is clear from context - or simply explicit - that this is an event in your past and her future, because the choice of tense is not going to elucidate this.
When using relative terms, there are no hard rules:
Context is everything in interpreting such statements. But note that these problems are not entirely without precedent. Consider the ambiguity of the term "yesterday" when used after flying across the International Date Line. (That's not to say that these problems are solved; even very carefully-written calendar software can trip up when, for example, scheduling a regular event to occur at 1:30am daily, just before Daylight Saving Time begins.)
The solution is to avoid ambiguous terms. Say "my yesterday" or "your next year" or "24 subjective hours ago" for absolute clarity.
First person pronouns should always be used to refer to your current, present self: the darkness behind your eyes. This becomes criticially important when, due to time travel, there are other versions of you wandering around.
In 1990, you tie your shoelaces. You travel back to 1957. You then wait until 1990 and go and find your past self. Through a pair of binoculars, from across the street, you observe your past self tying his or her shoelaces.
Do not refer to that individual as "myself" and do not smugly announce "I am tying my shoelaces" to third parties. Other versions of you must be referred to using the third person perspective, as in "My past self is tying his/her shoelaces". You may also number the various instances of yourself according to where in their personal timeline this event is taking place. In this example, if your name happened to be "Henrietta", then this would be "Henrietta-1 is tying her shoelaces". You, observing Henrietta-1 through the binoculars, are Henrietta-2.
If Henrietta-1 happened to look up and spot you, she could use the same rules: "Henrietta-2 is watching me tie my shoelaces". She would also be permitted to refer to you has her "future self" and she would not be permitted to say "I am watching myself tie my shoelaces".
It's important to understand how ambiguous sentences like these are without this convention! Remember that with time travel, a person can tie his or her past or future self's shoelaces! So, even with just two Henriettas present, the sentence "I am tying my shoelaces" has four distinct interpretations:
and "I am watching myself tie my shoelaces" has eight:
Of course, "I tied my shoelaces" is always correct; this event is still in your personal past.
In more complicated time travel models, history becomes malleable and/or divergent. It may be possible to change history by going back in time; it may be possible to create a new timeline by going back in time. The situation may be even more complex. The best example of this is the Back To The Future series.
In situations like this, we have to abandon another assumption: even the universe itself no longer has an objective past or future. Some of the conventions we came to rely on above are no longer useful.
The use of the present tense to refer to events is still acceptable. You may still say "The President of Malawi is pied in the face in 1990", provided that (1) you add a clarification as to the specific timeline or timelines in which this event appears or (2) this is clear from context. Remember, it is possible to modify history to avert this pieing, or delay it until 1991: it need not occur in 1990 in all timelines.
Use greater care when referring to events in the future-- anybody's future. History is malleable. Nothing is guaranteed to happen the same way every time. All you can make are educated guesses. (This is exactly like time-travel-free reality.)
Use greater care when referring to events in the past-- anybody's past. The day that a protester threw a custard pie in Malawi President's face, an event of international significance, may be erased from history so that only you remember it. It no longer matters that you weren't present or responsible: this event is now personal to you, and must be described as being part of your subjective past.
If history changes radically, you may find that this is true of every event that has ever directly or indirectly affected you - from the outcome of wars and assassinations to your own birth.
Remember the Three Things To Be:
Probably the most aggravating thing to do in a time travel conversation is to complain that conversations about time travel are difficult. Firstly, time travel grammar challenges everybody equally; your conversation partner is probably having just as much difficulty understanding you as you are having in expressing yourself unambiguously. As with every conversation involving specialist terminology, "English, please?" is simply insulting, and certainly isn't going to move you both closer to mutual understanding.
Secondly, time travel grammar isn't difficult unless you make it difficult for yourself. Put it like this: No matter how far you travel through time and how frequently you cause history to diverge or interact with your past and future selves, your experiences will always be linear. There will be a series of events leading up to your present situation, and there will be a series of events following it. What you can remember of your past might be confusing; you might not know anything concrete about your future. Oh, and other people's experiences might differ from your own. Shocking, right?