Present perfect tense
subject + verb to have (present tense) + past participle
subject + verb to have (present tense) + been + verb -ing
General use of the present perfect
The present perfect tense links the past to the present. There are three common ways in which it is used:
1) (Up until now)
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.
2) (Life experience)
To refer to something that happened at an unspecified time in the life of someone who is alive now.
She has been to Australia.
This means she went to Australia at sometime in her life. We do not know when or how many times.
3) (Present relevance)
To refer to something that happened in the past which has some importance or relevance now.
I've lost my glasses.
This means I lost my glasses at sometime, probably quite recently. The present relevance is that I don't have my glasses now.
For more detailed notes on each of these concepts, see the links below.
Present perfect versus past simple
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.
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).
For more detailed notes on how the past simple is used, see the link below