Present perfect tense

The PRESENT PERFECT TENSE is formed with a present tense form of "to have" plus the past participle of the verb (which can be either regular or irregular in form). This tense indicates either that an action was completed (finished or "perfected") at some point in the past or that the action extends to the present:
I have walked two miles already [but I'm still walking].
I have run the Boston Marathon [but that was some time ago].
The critics have praised the film Saving Private Ryan since it came out [and they continue to do so].
The choice between Present Perfect and Simple Past is often determined by the adverbial accompanying the verb. With adverbs referring to a period gone by, we would use the simple past:
I studied all night/yesterday/on Wednesday.
With adverbs beginning in the past and going up to present, we would use the present perfect:
I have studied up to now/lately/already.

Did you know that Has and Have is a characteristic of Perfect Tense.

Has / Have means already or already. So, Has or Have must be used if you want to make the Present Perfect Tense sentence.
When is Has / Have used? To answer this question, you need to know the subject well. Yes, Has / Have used according to the subject.

Therefore, we study the subject in the previous point.
HAS for Singular or one object.
HAVE for Plural, aka several or many objects.