Present perfect

The present perfect tense is a perfect tense used to express action that has been completed with respect to the present. (It is considered a present tense, not a past tense, since the resulting state is in the present.) "I have finished" is an example of the present perfect. The Present Perfect is a compound tense; it is formed by using the present tense of "have" ("have" or "has") and the past participle of a verb. In the above example, the past participle "finished" is the main verb, while "have" is the auxiliary verb.

This construction is one of the hardest points of grammar for people to understand. It is used to refer to a subject's past actions or states while keeping the subject in a present state of reference or in a present state of mind. Think of the words in the construction separately: "have" (or "has") is in the present, and the past participle is in the past. For example, "I have gone to the cinema" implies that the subject has completed a certain action (this is what "gone" relates), but that the subject is, in a sense, "holding" or "possessing" that completed action in the present time (this is what "have" relates). In other words, the subject is in a current state (now), and a past action that the subject has done or a past state that the subject has been in, is being referred to from the current state of the subject, which is the present time. This differs from the simple past tense, i.e., "I went to the cinema", which implies only that an action happened, with the subject having no relationship at all to the present.

Another example:

The boy saw the car. (Emphasis is on the fact that the boy saw the car.)

The boy has seen the car. (Emphasis is on the present state of the boy, resulting from the fact that he saw the car.)

I left Argentina eight years ago.

I have left Argentina for now.

In summary, both the present perfect tense and simple past tense are used for past actions or states, but the present perfect describes the present state of the subject as a result of a past action or state (i.e., the subject is being talked about in the present), whereas the simple past describes solely a past action or state of the subject (i.e., the subject is being talked about in the past).

In other words, it places the subject in the result phase of the event.

In other languages
In many European languages, including German, French and Italian (notably in the North), the present perfect has usurped the role of the simple past in spoken language, and the simple past is now used only in written form. In English, in contrast, the present perfect and simple past are kept distinct, although in sloppy speech sometimes the simple past is used where the present perfect would likely be more appropriate.

Lesson plan
lesson plan for present perfect to indicate change