* You use the present perfect (`I have walked’) to relate the past to the present.
* You use the past perfect (`I had walked’) to talk about a situation that occurred before a particular time in the past.
1 You use the present perfect tense when you are concerned with the present effects of something which happened at an indefinite time in the past.
I’m afraid I’ve forgotten my book.
Have you heard from Jill recently?
Sometimes, the present effects are important because they are very recent.
Karen has just passed her exams.
You also use the present perfect when you are thinking of a time which started in the past and is still continuing.
Have you really lived here for ten years?
He has worked here since 1987.
You also use the present perfect in time clauses, when you are talking about something which will be done at some time in the future.
Tell me when you have finished.
I’ll write to you as soon as I have heard from Jenny.
2 When you want to emphasize the fact that a recent event continued to happen for some time, you use the present perfect continuous.
She’s been crying.
I’ve been working hard all day.
3 You use the past perfect tense when you are looking back from a point in past time, and you are concerned with the effects of something which happened at an earlier time in the past.
I apologized because I had forgotten my book.
He felt much happier once he had found a new job.
They would have come if we had invited them.
You also use the past perfect when you are thinking of a time which had started earlier in the past but was still continuing.
I was about twenty. I had been studying French for a couple of years.
He hated games and had always managed to avoid children’s parties.
4 You use the future perfect tense when you are looking back from a point in the future and you are talking about something which will have happened at a time between now and that future point.
In another two years, you will have left school.
Take these tablets, and in twenty-four hours the pain will have gone.
You also use the future perfect when you are looking back from the present and guessing that an action will be finished.
I’m sure they will have arrived home by now.
It’s too late to ring Don. He will have left the house by now.
5 You can also use other modals with `have’, when you are looking back from a point in time at something which you think may have happened at an earlier time.
I might have finished work by then.
He should have arrived in Paris by the time we phone.
For more information on modals with `have’, see Units 79 to 91.