When do we use had has have?

Have and has are the same verb in different forms. Use have if the subject of the sentence is I, or a plural pronoun or a plural noun. eg

I have a red car.

They have a blue car. -- they= plural pronoun

The boys next door have two red cars. -- boys = plural noun

Use has if the subject of the sentence is singular. eg

She has a blue car. -- she = singular pronoun

The girl next door has a green car. -- girl = singular noun

Had is the past tense of have/has. Use it for sentences about the past. Use it for all subjects eg

I had a great day. -- I = a singular pronoun

They had a lousy day. -- they = plural pronoun

The boy had a great day -- boy = singular noun

The doctors at the conference had a good time -- doctors = plural noun