Hi,
Base form Past simple Past participle Present participle
Have Had Had Having
Example - I have a car (present)
Example - I had a car (in the past)
Correct usage of have and has:
I have, You have, We have, They have / She has, He has, It has
The guideline for correct usage of has and have is above in bold, it would be incorrect to say I has a car, as it would be incorrect to say She have a car.