“Defining The Moment” For The Past And Future Perfect And Continuous And Perfect Continuous
When we use the present tense in any combination of simple and complex tenses, the moment in time that we’re talking about is very clear – right now, at the moment spoken.
When we use past and future tenses, however, the moment we’re talking about is not clear. Adverb or adverb phrases can be used to tell us this, but all we know from past and future tense constructions is that their verbs happened or were before or will happen or be later.
I surfed once.
I’ll try to remember that.
As long as we stay in the simple, adverb phrases give enough information for complete meaning. A lot of the time we don’t need to know when, or can guess well enough, and if we do need to know, the speaker can tell us.
It isn’t enough though if we use either the perfect or the continuous tenses. The reason is that both of these tenses are relative, meaning that they compare two time periods. The perfect says that we have a result at some moment because of activity of the verb before that moment, and the continuous tells us that the verb was happening at some particular moment and was longer than that moment.
Unless we’re in the present, when the base time is obvious (now), we need to define the base time somehow because it’s too confusing for people to try to think about a complex time relationship to an unknown time or event. It might be the closest thing in language to the situation in algebra where you have too few equations for multiple unknowns.
a + b = 7
b = ?
There’s no way to know this. We have to SET the perfect and continuous in time somewhere, and if you aren’t in the present tense, you do that by defining the moment.
There are two ways to define the moment for the past and future perfect and/or continuous.
One is just to state the time.
I was sleeping at 3 a.m. last night.
By 5 a.m. last night I had slept for 7 hours.
I will be sleeping at 3 a.m. tonight.
By 5 a.m. tonight, I will have slept for 7 hours.
The other is to describe something that happened in the past or will happen in the future.
I was sleeping when the sun came up.
I had slept for 7 hours when the sun came up.
I will be sleeping when the sun comes up.
I will have been sleeping for 8 hours when the sun comes up.
Notice that the events in the past sentences are given in the past simple, while the ones in the future sentences are given in the present simple. Why is that?
The answer is that there isn’t truly a future tense in English, only constructions that mean the future, which can be put in either the present or past [if past, then future perfect].
Note that whether you need to define the time in your speech or writing depends on the context. It only has to be defined somehow, and this can be by another person or in any other way from the circumstances.