The perfect tenses

1.Present
1.1. The Present perfect
It expresses:

An idea that something happened (Or never happened)
before now at an unspecified time in the past. The exact time
when it happened is not important.
A repetition of an activity before now. The exact time of 
each repetition is not important.

If the time is important : Use the simple past.
A situation that began in the past and continues to the 
present (until now). Use since and for in this case.
Examples:
I have finished the work.
She has seen it many times.
We  have expected to get the response for two weeks.
They have expected to get the response since last Monday.

 
1.2. The Present perfect continuous
It expresses:

A duration of an activity that began in the past and 
continues to the present. 
When it has this meaning. It Is used with times words as 
since and for, all morning , all day, all week.(until now)

A general activity in progress recently, lately 
without any specific time.
 (The moment is: "RECENTLY" or "LATELY" Not the PRESENT  
as in the present simple)
Examples:
I have been experiencing this difficulty for ten years.
This program was interesting.
 

 
2. Past
2.1. The past perfect:
It expresses:

an activity that was completed before another activity 
or time in the past. It Is used with times words.
Examples:
He had lived in Virginia.
They have worked in the same office.
If either "before" or "after" is used in the sentence, the 
simple past may be used instead of the past perfect.
Examples:
I saw him before she arrived.
He had a coffe after the break.



2.2.The past perfect continuous 

Emphasizes the duration of an activity that was in 
progress before (until) another activity or time in the past.
Express also an activities  in progress recent to 
( before) another time or activity in the past.

She had been living in Virginia.
They had been working in the same issue.
 


3. Future
 
3.1. The future perfect
It expresses:

 an activity that will be completed before anther time 
or event in the future.
Example:
I will have finished this work before the phone rings
Before (or by the time) introduces a “time clause” where 
the simple present is used.


3.2. The future perfect continuous:

It emphasizes the duration of an activity that will be 
in progress before ( until) another time or event in the future.
Example:
She will have using all her money to play in this game.
I will have been sleeping for three hours before the phone rings.
Sometimes the future perfect and the future perfect continuous 
give the same meaning. (the activity expressed by either of these 
two tenses may begin in the past)

She will have stayed for three weeks in this city when she leaves 
this weekend.

She will have been staying for three weeks in this city when she 
leaves this weekend.