Present Perfect

Present Perfect
S + HAVE/HAS + Past Participle (V-ed or irregular)

+ Already started action
   _ I have studied for 3 months
   _ Mary has been with us for 6 months.
   _ The teacher has taught English since 1989.
   _ The teacher has taught us since he graduated from the university.
(without a definite time given. The recentness may be indicated by just)
+ Already completed action
   _ I have traveled to Ha Noi once
   _ I have seen John.
   _ He’s graduated from high school.
(which happened at an unknown time in the past. No definite time is given)

Time expressions with present perfect:
G1: just, ever, never, already
G2: yet, up to now, until now, till now, up to the present, so far
G3: once before, twice before, three times, several times before
G4: for + a period of time, since + a point of time

a. Experience: (co bao gio chua?)
Have you ever ……… (before)?

b. Accomplishment: ( …. Xong chua?)
Have you ……. Yet?

c. How long: (bao lau roi? từ trước đến giờ)
How long have you …….?

d. How many times: (….. may lan roi? từ trước đến giờ)
How many times have you …. ?

Choice between past simple and present perfect for recent events may depend on the attitude of the  speaker. This in turn may depend on whether the speaker feels distant in time or place from the event.
   _ I've left my wallet in the car. I'm going back to get it.
Here the speaker may be about to return, and feels that the event is connected with the present.
   _ I left my wallet in the car. I'm going back to get it.
The speaker may feel separated in time from the event, or be further away.