Improving Your Writing
Agreeing means that if the subject of the verb is singular, the verb must have a singular form; likewise if the subject of the verb is plural, the verb must have a plural form. For example, we say he sings (singular subject, singular verb form) and they sing (plural subject, plural verb form). He sing and they sings are incorrect because they mix singular and plural forms.
This example is a very easy one, but sometimes problems with agreement can be harder to spot.