Third person singular -s

Use the -s form of a verb in the present tense when the subject is third person singular.
For all other subjects, use the base form in the present tense.

Example:

base form
I live in Athens.
They live in Crete.
The Smiths live in Rome.

-s form
He lives in Cyprus.
She lives in Malta.
Tim lives in Naples.
Ms. Conner lives in Milan.

The verb have

The -s form of the verb have is has.

We have a winner.

He has a trophy.

The verb be

The -s form (present tense) for be is is. For the past tense, it is was.
Was is also used with first person singular subjects (I) in the past tense.
(Am is used in the present tense. Are is used with other subjects in the present, and were is used in the past.)

She is here.
He is not here.
I am here too.
You are right.
They are wrong.

She was home yesterday.
Ron wasn't home yesterday.
I was here yesterday.
We were here too.

Auxiliary verbs

When auxiliary verbs are used, only the first verb carries the third person singular -s.

Jean does not like spaghetti. (Correct)
Jean does not likes spaghetti. (Wrong!)
Jean do not likes spaghetti. (Wrong!)

Elsa is eating her dinner. (Correct)
Elsa is eats her dinner. (Wrong!)
Leo is going to eat later.

Frank has eaten lunch already. (Correct)
Frank have eaten lunch already. (Wrong!)

Modals

Do not use the -s form of the verb with modals, regardless of the subject.

Ian can eat twelve bowls of rice. (Correct)
Ian can eats twelve bowls of rice. (Wrong!)

See also:
Grammar: Verb List
Grammar: The Verb "Be"

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