Question

cuando se usa "have" y "has"

Answer 1

Presente Pasado Futuro
I have I had I will have
you have you had you will have
he/she/it HAS he/she/it had (Igual )
we have we had we will have
you have you had you will have
they have they had they will have

como verás las únicas personas que cambian en el presente simple, son el, ella y eso, una oración de ejemplo puede ser

I have a PC, (yo tengo una PC)
he/she/it HAS a hat (el/ella/perro) tiene un sombrero
We have a nice home (nosotros tenemos una casa agradable)
El tiempo pasado es igual para todas las personas y lo mismo con el futuro siempre anteponiendo el auxiliar WILL.

I have a car
He HAS a car

you have a cell phone
she HAS a cell phone

they have many balls in the garden
it(dog) HAS many balls in the garden

Espero te ayude

Answer 2

Has y Have son diferentes personas de un mismo verbo.
I have
You have
She/He/It has
We have
You have
They have