When do I use HAVE and HAS?

"Have" and "has" are both present tense conjugations of the verb "to have," and we use "have" or "has" depending on the subject.  

If the subject is 3rd person, singular, then you use "has."  All other subjects take on "have".  See below:

Singular                                 

1st Person         I have            

2nd Person        You have            

3rd Person       He/She/It HAS        

Plural
1st Person          We have
2nd Person         You have
3rd Person         They have

Ex.

She has a doctor's appointment tomorrow.

They have a long day ahead of them.

My dog has a good temperament.

We both have an iPhone.

My sister has a hybrid car.

Remember that the verb "to have" serves two purposes in the English language:

1. Verb: Of course, as you know, the verb "to have" is used to express possession, as in the examples above and as in "I have a dog, a house and a beautiful family."
2. Auxiliary Verb: "Have" is also used as an auxiliary (or helping) verb when using the present perfect.  (Click here to learn more about the present perfect.)  See below for the grammar construction of the present perfect:

Present Perfect Grammar:

When using the present perfect, we conjugate the verb "to have" in the same way as the present tense as seen above; that is, the 3rd person singular changes to "has."

I/You/We/They + HAVE + past participle

He/She/It            + HAS    + past participle

Ex. 

I have traveled to Paris three times.

He has never seen a James Bond movie!

They have taken the test.

She has tried to help.

We have just finished writing the report.

My dog has already gone for a walk four times today.

They have taken the test.

She has tried to help.