As you may know, have and has are both present forms of the verb to have and are employed with the verb have alone (for a present simple), with have/has (used as an auxiliary verb) and the past participle of another verb (for a present perfect) or with have/has (used as an auxiliary verb), the past participle of be and the past participle of another verb (for a present perfect continuous).
- Has is used with a third singular person ("he", "she", "it", or a noun) i.e. when the subject is singular but it's neither the person who's writing/speaking ("I") or the person who's reading/listening ("you"):
• He has just asked a question.
- Have is used in any other case:
• Can anyone tell me when we have to use "have", etc.
• I have just copied a sentence from your question.
• Have you noticed it?
Here you will find the complete conjugation of the verb.