The present / past perfect

[ath111]

We use the present perfect in two cases: First,when the action happened in the past but we don't know

exactly when,or let's say when the action exactly happened is not important.What is important is the action

Here,we use words like: before,already,just,never,yet

Example: I have visited Algiers before

The action of visiting Algiers is more important than when I visited it.I visited Algiers in the past

but when in the past? We don't know.Maybe yesterday,maybe last week,maybe last year,maybe

Second,we can use the present perfect when the action started in the past and continues in the present

Here,we use the two connectors: since and for

Example: I haven't seen my friend since last year

I haven't seen my friend for a year

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When we have two successive ( one after another ) actions in the past,we put the verb of the

action that happened first in the past perfect and the verb of the action that happened next

in the past simple

Example: After she had written the letter,she posted it

So,the action of writing happened before the action of posting.So,we put the verb write in the past

perfect and the verb post in the past simple