Present Perfect Tense

The present perfect tense describes a past event that has present tense implications (compare the simple past "I cooked twice this week" with the present perfect: "I have cooked twice this week" - the former implies that that's all the cooking I'm going to do, while the latter suggests that I might cook more).

English forms this tense with a combination of the auxiliary verb "to have" and a past participle: "I have noticed that," "She has gone there twice," etc. The name "present perfect" reflects the fact that the auxiliary verb, "to have," is conjugated in its present-tense forms.
Some verbs long ago used "to be" as the auxiliary, and we find vestiges of this structure in biblical English or old nursery rhymes:
The Lord is risen.
For He is come.

Elsie Marley is grown so fine, she won't get up to feed the swine, but lies in bed till eight or nine.
Weak and strong verbs form past participles differently.
Weak verbs add "-ed": "looked," "wanted," "worked," etc.
Strong, or irregular, verbs generally (but not always) change the vowel in the stressed syllable and typically (but not always) add "-n" or "-en": "risen," "written," "eaten," "sung," "slept," "brought," etc.