In academic varieties of English, verbs must agree with their subjects in number (singular or plural) and in person (first, second, or third).

To make a verb in the present tense agree with a third-person singular subject, add -s or -es to the base form.

A vegetarian diet lowers the risk of heart disease.

To make a verb in the present tense agree with any other subject, use the base form of the verb.

I miss my family.

They live in another state.

The verbs have and be do not follow the -s or -es pattern with third-person singular subjects. Have changes to has; be has irregular forms in both the present and past tenses and in the first person as well as the third person. (See Verbs.)

War is hell.

The soldier was brave beyond the call of duty.

In some varieties of African American or regional English, third-person singular verb forms do not end with -s or -es: She go to work every day. In most academic writing, however, your audience will expect third-person singular verb forms to end in -s or -es.