-that are completed when the time is not important. There is a connection with now.
-that started in the past and are still true now.
I've stayed at that hotel before so I know what it's like. She's worked in Sydney so she knows what to expect.
I have lived here since 1989.
This form is used with already, just and yet
They have lived already been to the sahara
He's just heard the news. Have you heard about it yet?