Past Perfect emphasizes that we are talking about a period before a time in the past. If the time sequence is clear (e.g. because we use after), both Past Simple and Past Perfect are possible: I got to work after Simon arrived / had arrived.
At other times Past Perfect is essential to understanding the sequence, and we often add already, as soon as, or until: When I arrived, they had already started (= they started before I arrived) When I arrived, they started. (= I arrived before they started)
With definite time
Unlike Present Perfect, we can use Past Perfect with a definite time reference: I arrived at nine o’clock but he had got there at eight.
There is no exception to the time sequence rules on Past Perfect. When we use before, the verb in Past Simple can refer to something that takes place before the verb in Past Perfect. The first action may prevent the second from happening: The waiter took my plate away before I had finished eating. I was blamed for it before I had even had a chance to defend myself.
We use Past Perfect with report verbs and with hope, intend, expect, etc. to talk about plans that have not yet been fulfilled. Had is usually stressed in speech with this use: I had hoped to talk to him but he was too busy. I had thought of phoning him but decided against it.
2. FUTURE PERFECT
We often use Future Perfect with the preposition by or the phrase by the time meaning ‘at some point before the time mentioned’: It’s taking her so long to write that book that by the time she’s finished it people will have forgotten the incident it’s based on.
We can also use will have done to say what we think has probably happened: There is no point phoning: they will have gone out.
We can use should / ought to or may / might instead of will if there is some uncertainty about the prediction of present or future: I should have finished making this cake by the time Sue comes home. (= I think I will have, but I am not sure)
3. USES OF PERFECT INFINITIVES
We use Perfect infinitives:
after link verbs like seem and appear to refer to a previous time period (an ordinary to-infinitive will usually refer to the present or future) There seems to have been some sort of mistake.
after phrases expressing emotions and feelings: I’m sorry to have kept you waiting. She was felt not to have met the standards required.
4. PERFECT -ING FORMS
We can use Perfect -ing form to emphasize that one thing happens before another: I didn’t remember having met / meeting her before. Having finally grasped what I meant, he got down to work.