In everyday terms, the word agreement refers to an accord of some sort: you reach an agreement with your boss about salary; friends agree to go to a movie; the members of a family agree to share household chores. This meaning covers grammatical agreement as well. 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.

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 tense and the past tense.

War is hell.

The soldier was brave beyond the call of duty.