When you want to talk about results following a past, present, or future action, you need to use the perfect aspect.
In order to look back from the present, we need Present Perfect. It also describes states and long actions which started in the past and continue until the present:
Ann and Jack have been married for more than 20 years.
We use Present Perfect if we need to talk about somebody’s experience:
Dave has ridden an elephant.
We also use Present Perfect when the result of the past action is still relevant:
I’ve shared your secret with Debby. I’m really sorry but she knows everything now.
Past Perfect is used when we need to talk about something that happened before another action in the past, especially when we are presenting this sequence of actions in a non-chronological order:
Mark suddenly bumped into the bags that he had left in the hall.
We also use Past Perfect to talk about an earlier action, the result of which was still relevant at a later stage:
My mum had cooked dinner for me, so there was no need to order a takeaway.
Future Perfect usually describes actions which will be completed before a later point in the future:
The redecoration will have finished by the end of this week.
It is important to note that we never state the exact time when the action happens; instead, we use adverbs such as just, never, ever, never, since, for, before, yet, already, until. Some of them are often used in mid-position (i.e. between the auxiliary verb and the main verb), whereas others usually come at the end of the sentence:
Jane has never worked as part of a team before.
When we came to the cinema, the film had already started.
The committee won’t have made a decision until the next meeting.
The passive forms are:
This tricky situation will have been solved by Monday.
We didn’t have a chance to try the cake: it had been eaten before we came home.
The check-up has just been finished; you can go home.
We use perfect infinitives to refer to things that might have happened in the past, usually after such verbs as: to claim, to expect, to hate, to hope, to love, to like, to pretend, to prefer, to be sure, to appear, would like:
Mark pretended to have forgotten his homework at home.
I would prefer to have stayed on my own for some time.
We can also use the perfect infinitive with future reference:
The government hopes to have finished the repair works of the road by the end of the week.
In order to talk about unreal situations, one can use the perfect infinitive after modal auxiliary verbs: could, would, might, ought, should and needn’t:
You could have told me about your cat! Don’t you know that I’m allergic to animals!
We also refer to the past and say how confident we are that something has happened:
We have nothing to eat. Mike might have taken all our food. (speculation)
Look at all that snow! It must have snowed all night! (confidence)
And now it’s high time to put all the rules you have learned into practice!