When the present perfect is used with since or for, it expresses situations that began in the past and continue to the present.
In (f): Class started at ten. We are still in class now, at the moment of speaking.
Incorrect: We are in class since ten o’clock this morning.

4-2 Simple Past vs. Present Perfect

Simple Past:
(a) I finished my work two hours ago.
Present Perfect:
(b) I have already finished my work.

Simple past:
(c) I was in Europe last year/three years ago/in 1999/in 1995 and 1999/when I was ten years old.
Present perfect:
(d) I have been in Europe many times/several times/a couple of times/once/(no mention of time).

Simple Past:
(e) Ann was in Miami for two weeks.
Present perfect:
(f) Bob has been in Miami For two weeks/since May first.

In (e): In sentences where for is used in a time expression, the simple past expresses an activity that began and ended in the past.
In (f): In sentences with for or since, the present perfect expresses an activity that began in the past and continues to the present.

Since

                                     since eight o’clock.
                                     since  Tuesday.
                                     since  May.
(a) I have been here     since  1999.
                                    since  January3, 2001
                                    since  yesterday.
                                    since  last month.                                     

Since is followed by the mention of a specific point in time: an hour, a day, a month, a year, etc.
Since expresses, the idea that something began at a specific time in the past and continues to the present.

(b) Correct:     I have lived here since May.
      Correct:    I have been here since May.
(c)  Incorrect:  I am living here since May.
(d)  Incorrect:  I am living here since May.
(e)  Incorrect:  I lived here since May.
      Incorrect:  I was here since May

The present perfect is used in sentences with since.
In (c): The present progressive is NOT used.
In (d): The simple present is NOT used.
In(e): The simple past n NOT used.

       Main clause           Since-Clause
     (present perfect)      (simple past)
(f) I have lived here.    since I was a child.
(g)   Al has met many people        since he came here.

For

                                         for ten minutes.
                                         for  two hours.
                                        for  five days.
(h) I have been here        for  about three weeks.
                                       for  almost six months.
                                       for   many years.
                               for   a long time.

For is followed by the mention of a length of time: two minutes, three hours, four days, five weeks, etc.
Note: If the noun ends in-s (hours, days, weeks, etc.), use for in the time expression, not since.

      Al and Ann are in their car tight now. They are  
      driving home. It is now dour o’clock.     
(a) They have been driving since two o’clock.
(b) They have been driving for two hours.
      They will be home soon.        

(d) Po is sitting in class right now.

The Present Progressive describes an activity that is in progress right now, as in (d). it dose not discuss duration (length of time).
Incorrect: Po has been sitting in class right now.

4-5 Present Perfect Progressive vs. Present Perfect

Present Perfect Progressive
(a) Rita and Josh are talking on the phone.
  They have been talking on the phone for
    twenty minutes.     

Present Perfect
(b) Rita has talked to Josh on the phone many times. (before now).
(c) Incorrect: Rita has been talking to Josh on the phone many times.
(d) Rita has known Josh for two years.
(e) Incorrect: Rita has been knowing Josh for two years.

(2) the duration of present situations, as in (d), using non-action verbs.

(f) I have been living here for six months. OR
(g) I have lived here for six months.
(h) Al has been wearing glasses since he was ten.   OR
     Al has worn glasses since he was ten.
(i) I've been going to school ever since I was five years old. OR
   I've gone to school ever since I was five years old.

For some (not all) verbs, duration can be expressed by either the present perfect or the present perfect progressive. (f) and (g) have essentially the same meaning, and both are correct.
Often either tense can be use with verbs that express the duration of usual or habitual activities/situations.

Already

(a) The mail came an hour ago.
       The mail is already here.                     

Idea of already: something happened before now, before this time.
Position: midsentence.

Yet

Idea of yet: Something did not happen before now (up to this time), but it may happen in the future.
Position: end of sentence.

Still

(c) It was cold yesterday. It is still cold today.            We still need to wear coats.
(d) I could play the piano when I was a child. I can still play the piano.  
(e) The mail didn't come an hour ago. 
      The mail still hasn't come.                  

Idea of still: situation continues to exist from past to present without change.
Position: midsentence.

 4-7  Past Perfect   

 Situation: Jack is left his apartment at 2:00.
Ann arrived at his apartment at 2:15 and knocked on the door.
(a) When Ann arrived, Jack wasn't there. He had left.

The Past Perfect is used when the speaker is talking about two different events at two different times in the past; one event ends before the second event happens.
In (a): There are two events, and happened in the past:
Jack left his apartment. Ann arrived at his apartment. To show the time relationship between the two events, we use the past perfect (had left) to say that the first event (Jack leaving his apartment) was completed before the second vent (Ann arriving at his apartment) occurred.

(b) Jack had left his apartment when Ann arrived.

Compare The Present Perfect and The Past Perfect: