How do we use the Present Perfect Tense?

This tense is called the present perfect tense. There is always a connection with the past and with the present. There are basically three uses for the present perfect tense:

1. experience
2. change
3. continuing situation

1. Present perfect tense for experience

We often use the present perfect tense to talk about experience from the past. We are not interested in when you did something. We only want to know if you did it:

I have seen ET. He has lived in Bangkok. Have you been there? We have never eaten caviar.
past	present	future
!!!
The action or state was in the past.	In my head, I have a memory now.

Connection with past: the event was in the past.
Connection with present: in my head, now, I have a memory of the event; I know something about the event; I have experience of it.

2. Present perfect tense for change

We also use the present perfect tense to talk about a change or new information:

I have bought a car.
past	present	future
-	+
Last week I didn't have a car.	Now I have a car.

John has broken his leg.
past	present	future
+	-
Yesterday John had a good leg.	Now he has a bad leg.

Has the price gone up?
past	present	future
+	-
Was the price $1.50 yesterday?	Is the price $1.70 today?

The police have arrested the killer.
past	present	future
-	+
Yesterday the killer was free.	Now he is in prison.

Connection with past: the past is the opposite of the present.
Connection with present: the present is the opposite of the past.

Americans do not use the present perfect tense so much as British speakers. Americans often use the past tense instead. An American might say "Did you have lunch?", where a British person would say "Have you had lunch?"

3. Present perfect tense for continuing situation

We often use the present perfect tense to talk about a continuing situation. This is a state that started in the past and continues in the present (and will probably continue into the future). This is a state (not an action). We usually use for or since with this structure.

I have worked here since June. He has been ill for 2 days. How long have you known Tara?
past	present	future
The situation started in the past.	It continues up to now.	(It will probably continue into the future.)

Connection with past: the situation started in the past.
Connection with present: the situation continues in the present