The present perfect tense is quite complicated to explain. It is used when an action that happened in the past continues to have a strong connection in the present. The best way to understand it is to look at some examples. They are followed in each case by a short explanation:
I have lost my dictionary.
(I don't have my dictionary now; can you help me find it?)
Mary has fixed my computer (My computer is working now and I'm happy about it!)
You haven't eaten very much. (Don't you feel well? Don't you like it?)
I haven't read his letter. (I haven't had time yet. What does he say?)
Have you seen my calculator? (- I want to use it now!)
Has she had an accident? (- Someone said she's in hospital!)
Have you done your homework? (- It's due today!)
With words of unfinished time
The present perfect tense is used with words or expressions of unfinished time. Unfinished time started in the past and continues into the present. (So you can see how this use of the present perfect is connected with use 1 above.) Here are some sentences in the present perfect. The expressions of unfinished time are shown in bold.
I've played tennis 3 times already this week and it's only Thursday!
She's been back to Korea twice already this year, and she's going again next week!
Sorry, I've seen that film already. I don't want to see it again.
I've lived in Germany since 1986.
She's had a lot of bad luck recently.
I haven't seen my mother for 2 months.
No, you can't use the bathroom. You haven't finished the exercise yet.
She hasn't called me lately? Do you think she's sick?
Have you ever seen a ghost?
Has he lived here all his life?
Has the postman been yet?
** Be careful: sometimes an expression of unfinished time can be used with the past simple tense. Look at these examples and explanations:
I drank three cups of coffee this morning.
The speaker is talking in the evening so for him this morning
is finished time.
I didn't see John today.
The student is talking after school when there is no more possibility of seeing John on this day. Today becomes an expression of finished time.
This year was a very good year for me!
This is possible if the speaker is talking very near to the end of the year, and so in her opinion the year is finished. Therefore she uses the past simple was.