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.
The verb have
The –s form of the verb have is has.
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.)
When auxiliary verbs are used, only the first verb carries the third person singular –s.
Do not use the –s form of the verb with modals, regardless of the subject.