Present Perfect Tense

a. Form: S + Have/ Has + Past Participle (PP)
b. Uses

• The present perfect looks back from the present into the past, and expresses what has happened before now. The action happened at an indefinite time in the past. Ex: I have met a lot of famous people - (Before now). She has won awards - (In her life).
• It expresses a past action that has a present result. The action is usually in the recent past. Ex: I have lost my key. The taxi hasn't arrived. He has written 25 books - (Up to now).
• The action can continue to the present, and probably into the future. Ex: she has lived here for 10 years - (She still lives here).
• The present perfect expresses an experience as part of someone's life. "Never, ever" are common with this use. Ex: I've travelled a lot in Africa. They have lived all over the world. Have you ever been in a car crash? My mother has never flown in a plane.
• The present perfect expresses an action or state, which began in the past and continues to the present. Ex: I have known Alice for 6 years. How long have you worked as a teacher?

Note: the time expressions for and since are common with this use. We use for with a period of time, and since with a point in time. Ex: we have lived here for 3 years - (A period of time). I've had a beard since I left the army.

• The present perfect expresses a past action with result in the present. It is often a recent past action. Ex: I have lost my wallet. (I haven't got it now). The taxi has arrived - (It's outside the door now).

Note: the adverbs "just, already, yet" are common with this use. "Yet" is used in questions and negative. Ex: she has just had some good news. I've already had breakfast. Has the postman been yet?
c. Present perfect and past simple
Past simple:

• Refers to an action that happened at a definite time in the past, and the action is finished. Ex: he died in 1889. She got married when she was 18.
• Time expressions + the past simple. Ex: I bought my car in 2000/ last year/ two years ago/ on March 20. I lived in Hanoi for a year (but not now).

Present perfect:

• Refers to an action that happened at an indefinite time in the past. The action can continue to the present. Ex: she has won awards. He has written 15 books. She has lived there for 20 years (and she still does).
• Time expressions + the present perfect. Ex: I have worked here for 3 years/ since 2002/ since I left school. We have never been to America.