Present perfect tense

The present perfect tense links the past to the present. There are three common ways in which it is used:
To refer to something that started in the past and is still happening now.

I have lived here for three years.
This means I moved here three years ago and I still live here.

We have been walking since nine o’clock.
This means we started walking at nine o’clock and we are still walking now.

To refer to something that happened in a time period that is unfinished (today, this week, this year, or in the life of a living person). The exact time is not mentioned.

She has been to Australia.
This means she went to Australia at some time in the past. We do not know when or how many times. She is still alive.

Have you ever seen a snake?
This question is asking, ‘Did you see a snake at some time in your life?’

I have drunk two coffees today.
This means I drank two coffees earlier today.

We haven’t been abroad this year.
This means we didn’t go abroad at any time this year.

To refer to something that happened in the past which has some importance or relevance now.

I've lost my glasses. Have you seen them anywhere?
This means I lost my glasses at sometime, probably quite recently. It is relevant because I don't have my glasses now.

There’s been an accident on the motorway. It was on the news. We’d better take the other road.
This means there was an accident on the motorway recently. Therefore, it would be best to avoid this road now.

The present perfect is not used to talk about things that happened at speficic times in the past, are finished, or do not have a direct relevance now. In these cases the past simple is normally used.

I lived in London for three years when I was a student.
This means I lived in London for a period of three years at a specific time in the past - when I was a student. I do not live in London now.

I went to Australia last year.
This means I went to Australia at a specific time in the past. I am not there now.

My grandfather never went to Australia.
This means my grandfather didn’t go to Australia at any time in his life. He is no longer alive.

I drank three coffees yesterday.
This happened in a time period in the past that is now finished.

We went abroad twice last year.
This happened in a time period in the past that is now finished.

I lost my glasses when I was on holiday.
This means I lost my glasses at a specific time in the past. There is no present relevance (presumably you have bought a new pair of glasses).