Present perfect (summary)
We use the present perfect tense to talk about actions in the past with a result now, at the present moment. We are not speaking about when the action happened. It is important that the action started before the moment of speaking and it has some result at the moment of speaking.
The signal words of the present perfect are just, already, yet, since, for, ever, never.
To build the present perfect we use the auxiliary verb have/has and the past participle of the main verb (its third form):
have/has + past participle
haven't (have not)
hasn't (has not)