Relative and absolute tense

Grammatical tenses are deictic; that is, the time they refer to cannot be known without context. The center of deixis may be either the moment of discourse or narration (a so-called absolute tense) or the moment under discussion (a so-called relative tense).

English uses absolute tense. For example, if John told someone "I will go to the party." but doesn't show up, that person would report his words as "John said that he would come." Because the event took place in the past, all verbs must be in the past tense. (Would is the past tense of will.) The phrase "John said that he will come" means something different: That the time of his expected arrival is in the absolute future, later than the time of reporting it.

In a language with relative tense, however, both situations would be described as "John said that he will come." Because the time of arrival is later than the time of his words (that is, the moment when John actually came was some time after the moment when he was talking), the verb come must be in the future tense, regardless of whether it is past, present, or future of the current moment. In some treatments, the terms anterior tense and posterior tense are used for relative past and future.

A few authors (e.g. Joan Bybee) use the term 'anterior' for the perfect. However, while the perfect is anterior in tense, it also includes the aspectual distinction of being relevant to the time in question, and is therefore more than a simple tense.

In English

	Past reference time	Present reference time	Future reference time
Event before reference time (anterior)	I had written	I have written	I will have written
Event at reference time (simultaneous)	I wrote	I write/am writing	I write/will write
Event after reference time (posterior)	I would write	I will write	I will write

English can refer both to a reference time (past, present, or future) in which a situation takes place, and the time of a particular event relative to the reference time (before, at, or after).

Consider, for example, the following sentences:

• When I got home yesterday, John arrived and met me (past reference, simultaneous event).
• When I got home yesterday, John had already arrived (past reference, anterior event).
• When I got home yesterday, John called and said he would arrive soon (past reference, posterior event).

In a present frame of reference, the same sentences appear as follows:

• Whenever I get home, John arrives and meets me (present reference, simultaneous event).
• Whenever I get home, usually John has already arrived (present reference, anterior event).
• Whenever I get home, John usually calls and says he will arrive soon (present reference, posterior event).

The equivalent in a future frame of reference is as follows:

• When I get home tomorrow, John will arrive and meet me (future reference, simultaneous event).
• When I get home tomorrow, John will have already arrived (future reference, anterior event).
• When I get home tomorrow, John will probably call and say he will arrive soon (future reference, posterior event).

"Will" as an indicator of absolute time

Note that the future construction will arrive does not directly fit into the above system. What is important about it is that the event described occurs at an absolute time in the future, not whether it occurs simultaneous with or after another event. For this reason, the construction with will can be used even in a past frame of reference to indicate an event that is unambiguously in the absolute future. For example, "When I got home yesterday, John called and said he will arrive next week" is possible, and emphasizes that the arrival is in the future (relative to the present time) even if the frame of reference is in the past. ("... said he would arrive next week" is also possible.)

Note also that in certain circumstances, the present tense form can be used to express a future event. This generally occurs only when a future frame of reference is being established, not when the reference time is already established and the relative time of the event is being expressed. For example, all of the following are possible:

• "I will leave tomorrow"
• "I leave tomorrow"
• "I am leaving tomorrow"

In a subordinate clause introduced by when, if or whether, only the simple present tense form is possible even when referring to a future event (as in "When I get home tomorrow" but not "*When I will get home tomorrow" or "*When I am getting home tomorrow"). On the other hand, in the main clause following a subordinate when clause, after the frame of reference is already established, will usually must be used. ("When I get home tomorrow, John will arrive and meet me" but not "*When I get home tomorrow, John arrives and meets me".)

Time reference in conditional sentences

The referencing of time is very different in conditional sentences, e.g.:

• If I win the lottery tomorrow, I will buy all my friends Lamborghinis. (normal conditional, future reference)
• If I won the lottery tomorrow, I would buy all my friends Lamborghinis. (future contrary-to-fact conditional)

The second sentence has the apparently contradictory combination of the past tense form and explicitly future reference ("tomorrow"). In this case, the switch from present to past tense form does not affect the actual time reference of the sentence, but changes it to have a contrary-to-fact meaning, i.e. it adds the additional sense of "I probably won't win the lottery tomorrow".

With present reference, the tense form use is similar:

• If he buys a Lamborghini, he also gets a Porsche free. (normal conditional, present reference)
• If he bought a Lamborghini, he would also get a Porsche free. (present contrary-to-fact conditional)

In this case, the contrary-to-fact implication is stronger, specifically, he isn't (rather than probably isn't) buying a Lamborghini.

Finally, with past reference, all tense forms need to be moved farther into the past:

• If he bought a Lamborghini, he also got a Porsche free. (normal conditional, past reference)
• If he had bought a Lamborghini, he would also have gotten a Porsche free. (past contrary-to-fact conditional)

The sense of the first sentence is "I don't know if this thing happened, but if it did, then this other thing also happened." The second has the sense "This thing definitely didn't happen. But if it had happened, then this other thing would also have happened."