Question on Have, had, has

[farcus]

Can somebody please once and for all tell me what is the proper use for each? like I know had is used as past tense helping verb. I'm asking because SAT is coming up also what is a good way to remember idioms and stuff.

[Mister Micawber]

Have and has are present forms of the verb. Has is used for the third person only (s/he, it has).

Had is the past tense form for all persons.

The same rules apply whether have is the main verb or an auxiliary verb:

I have a sandwich.
She has a sandwich.
She had a sandwich.
I have eaten it.
She has eaten it.
She had eaten it when I arrived.
She has had a headache for the last three years.
She has been eaten by a velociraptor.
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Native English teacher at Mister Micawber's