1 The verb agrees with the subject in number and person.
Yo canto.
Tú bailas.
2 When several subjects appear, we prefer the agreement in first person of the plural instead of second; also we prefer the agreement in second person instead of third person.
Matilde y yo no nos conocemos.
Tú y Matilde os conocéis.
3 Sometimes, the copulative verbs agree with the attribute.
Esto son mentiras y calumnias.
4 With collective nouns (gente, multitud, gentío, muchedumbre, etc.) the verb may either be in singular or in plural, but we tend to the singular.
La gente es egoísta.
5 The verb of a subordinate relative clause may agree or not with the subject if the verb of the main clause is ser used in first or second singular persons. Popular Spanish tends to agree the subordinate verbs en third persons singular.
Yo soy el que vino ayer. popular
Yo soy el que vine ayer.
Tú fuiste quien me avisó. popular
Tú fuiste quien me avisaste.
With other persons of the verb ser is used the natural agreement.
Nosotros somos los que vinimos ayer.
Vosotros fuisteis quienes me avisasteis ayer.