Present Perfect

(a) They have moved into a new apartment.
(b) Have you ever visited Mexico?
(c) I have never seen snow.
(d) I have already seen that movie.
(e) Jack hasn't seen it yet.
(f) Ann started a letter to her parents last week, but she still hasn't finished it.
(g) Alex feels bad, He has just heard some bad news.
The present perfect express the idea that something happened (or never happened) before now, at an unspecified time in the past. The exact time it happened is not important.
If there is a specific mention of time, the simple past is used: They moved into a new apartment last month.
Notice in the examples: the adverbs ever, never, already, yet, still, and just are frequently used with the present perfect.

(h) We have had four tests so far this semester.
(i) I have written my wife a letter every other day for the last two weeks.
(j) I have met many people since I came here in June.
(k) I have flown on an airplane many times.
The presen perfect also express the repetition of an activity before now. The exact time of each repetition is not important.
Notice in (h): so far is frequently used with the present perfect.

(l) I have been here since seven o'clock.
(m) We have been here for two weeks.
(n) I have had this same pair of shoes for three years.
(o) I have liked cowboy movies ever since I was a child.
(p) I have known him for many years.
The present perfect, when used with for or since, also express a situation that began in the past and continues to the present.*
In the examples, notice the difference between since and for:
since + a particular time
for + a duration of time
*The verbs used in the present perfect to express a situation that began and still exist are typically verbs with a stative meaning (see Chart 2-3).
The present perfect progressive, rather than present progressive, is used with action verbs to express an activity that began in the past and continues to the present (see Chart 3-2):
I've been sitting at my desk for an hour. Jack has been watching TV since seven o'clock.