Learn English – Using simple future and future perfect

The difference is in the reference point of time where the speaker perceives him/herself to be.

I will do my homework by 6 p.m.

The time is sometime before 6pm, and the speaker is simply asserting an intention to do the homework before the stated time.

I will have done my homework by 6 p.m.

The time when this sentence is spoken is the same as before, but the speaker is moving the reference point to some time in the future after 6pm. Effectively s/he is saying that "When we next speak (at sometime after 6pm) to see what my homework status is, I will be able to say that I have done my homework".

The future perfect tense is useful when discussing something else that is to happen. For example

"Let's go to the cinema tonight. The movie starts at 7pm and it takes half an hour to get there. Can you come?"

"I think so. I will have done my homework by 6pm, which gives us half an hour to eat something before we need to leave. So yes, let's go."

Let's first talk about the following two sentences:

1- Sara went to bed as soon as she had finished homework.

2- Sara went to bed as soon as she finished homework.

I think your confusion is valid because we use the past perfect when we talk about something that took place before another thing in the past. So the use of the past perfect comes across in the first sentence but the use of the past simple in the second sentence doesn't. Am I right? In fact, we don't need to use the past perfect unless it is necessary or unavoidable to do so. Even if we talk about one action happening before the other one, it is possible to use the simple past for both actions if we think it is not necessary to highlight or emphasize the happening of the earlier action. It sounds natural to avoid using the past perfect where the simple past works, which is used to refer to something or several things happening in sequence (one after another) in the past.

So both of the sentences are grammatically correct. However, I'll prefer the second phrase to the first one.

As for the last two sentences, it is correct to say that "everyone had gone home when Sara got to the party", but it's not grammatically correct to say that "everyone had gone home when Sara had got to the party". It doesn't make sense. In the past perfect when we talk about two events, we use the simple past in one clause and the past perfect in the second clause.

Let's now talk about the following sentence you are confused about:

"Everyone went home when Sara had got to the party".

There is nothing wrong with this sentence, but the meaning is other way round. It means that first Sara got to the party and then every one went home. Look at the first sentence again. When Sara got to the party, everyone had gone home. Here it means that first everyone went home and then Sara got to the party. Sometimes, one action happens soon after the other action, here we should use the past simple in both clauses such as when Sara got to the party, everyone left, when they saw the police, they ran away, etc.

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