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Which Tense should I use to tell if completed action in the future?

e.g 1

a) Later we realized that he changed his route. (in the past context)

b) Later we realize that he change/changed his route. (in the future context)

e.g. 2

a) Suppose he changed his address in the book but later realized that it was wrong ( In the past Context)

b) Suppose in the future he changed his address in the book but later realized the it is wrong, can he change the same at that time? (In future context)

e.g 3

a) Later we come to know that they spent/spend a whole day in the computer fair. (in future context)

I am mainly concern about my construction in future context. Please help.

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Future perfect

The future perfect tense indicates completion in the future:

By tomorrow afternoon, he will have changed his route.
When I'm 65, I will have saved up a fortune.
In two more weeks, you will have seen every car in the museum.

However, this tense works like the present perfect: it puts the verb into a whole time interval ending at some time in the future. It's suitable for accumulations (like saving up money), repeated events, and actions completed at an unspecified time before a specified future time—hence the by in the first example. It doesn't make sense if you say a specific time in the future for the event:

At 3:00 p.m. tomorrow, he will have changed his route.

If you mean that the act of changing his route will occur at precisely 3:00 p.m., then you use the simple future tense:

At 3:00 p.m. tomorrow, he will change his route.

The future present

This sentence describes the future but grammatically it's in the present tense:

Later we realize that he changed his route.

This sentence means that the changing of the route was completed before you realized it, but it doesn't say whether the changing of the route happened before now or after now. The adverb Later with the present tense moves the present tense into the future.* From that future perspective, the changing of the route happened in the past, so it gets the simple past tense. The same is true if you put the main verb into the future tense:

Later we will come to know that they spent a whole day at the computer fair.

What we learn later might be that they spent the day at the computer fair yesterday, today, or next week; the sentence doesn't specify.

English tenses concisely provide a lot of information about the times when actions happen in relation to a reference point in time (like the present, or a specified time in the past or the future), but they're not so precise about absolute time. For that, you need to use specific words, like "tomorrow at 3:00 p.m.", rather than grammar alone.

* Later can even move the present tense into the past—a later point in the past than was just discussed. "Now let's get our story straight before we talk to the police. Two years ago, we think he took the 405 into Los Angeles. Later—that is, last year—we realize that he changed his route."

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  • Is it ok if we use past tense of verb (Changed/Spent) to describe future action? or just simple form can be used?
    – user4084
    Commented Oct 29, 2019 at 8:53
  • @user4084 You can use the simple present tense to describe future action, e.g. "Until we meet again," "In September, we perform in Chicago," "Tonight we find out who wins" but the meaning is special: a wish, a scheduled event, an idea for a story. You can use the past tense to describe future action, e.g. "If you came tomorrow, I would have more time," but the meaning is usually hypothetical. And also your examples illustrate the past tense for action that might be in the future.
    – Ben Kovitz
    Commented Oct 29, 2019 at 9:13

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