present perfect

We use the present perfect to show that something has continued up to the present

They’ve been married for nearly fifty years.
She has lived in Liverpool all her life.

… or is important in the present:

I’ve lost my keys. I can’t get into the house.
Teresa isn’t at home. I think she has gone shopping.

We use the present perfect continuous to show that something has been continuing up to the present:

It’s been raining for hours.
We’ve been waiting here since six o’clock this morning.

We use the past perfect to show that something continued up to a time in the past:

When George died he and Anne had been married for nearly fifty years.

... or was important at that time in the past:

I couldn’t get into the house. I had lost my keys.
Teresa wasn’t at home. She had gone shopping.

We use the past perfect continuous to show that something had been continuing up to a time in the past or was important at that time in the past:

Everything was wet. It had been raining for hours.
He was a wonderful guitarist. He had been playing ever since he was a teenager.

We use will with the perfect to show that something will be complete at some time in the future:

In a few years they will have discovered a cure for the common cold.
I can come out tonight. I'll have finished my homework by then.

We use would with the perfect to refer to something that did not happen in the past but would have happened if the conditions had been right:

If you had asked me I would have helped you.
I would have helped you, but you didn’t ask me.
You didn’t ask me or I would have helped you.

We use other modals with perfective aspect when we are looking back from a point in time when something might have happened, should have happened or would have happened.

The point of time may be in the future:

We’ll meet again next week. We might have finished the work by then.
I will phone at six o’clock. He should have got home by then.

the present:

It’s getting late. They should have arrived by now.
He’s still not here. He must have missed his train.

or the past:

I wasn’t feeling well. I must have eaten something bad.
I checked my cell phone. She could have left a message.