I haven’t been to the zoo ... I was 8 years old.
I haven’t been to the zoo since I was 8 years old.
Penser au point de départ de l'action.
It was ... the Christmas holidays.
It was during the Christmas holidays.
Penser au nom qui suit.
I bought a car 4 years ... .
I bought a car 4 years ago.
Penser au temps écoulé entre l'action et le moment où l'on parle.
I have been living in London ... October.
I have been living in London since October.
Penser au point de départ de l'action qui a commencé dans le passé et qui continue dans le présent.
It happened a hundred years ... .
It happened a hundred years ago.
L'action est passée et datée.
The teacher said ... the class that it was important.
The teacher said during the class that it was important.
On répond à la question "when?".
I've known Elena ... primary school.
I've known Elena since primary school.
On évoque une action non terminée au moment où l'on parle.
It rained ... three days.
It rained for three days.
L'action est terminée, il ne pleut plus.