We usually use a plural noun, when we are talking about several people each doing the same thing.

All students should bring their drawing books to school tomorrow. (NOT All students should bring their drawing book to school tomorrow.)

Eight people lost their lives in the explosion. (NOT Eight people lost their life in the explosion.)

Note that uncountable nouns cannot be used in the plural.

We are all eager to increase our knowledge. (NOT We are all eager to increase our knowledges.)

When we talk about repeated single events, we often use plural nouns.

Do you often get headaches? (NOT Do you often get a headache?)
We sometimes go for rides over the hills. (More natural than ‘We sometimes go for a ride over the hills.’)

In generalizations both singular and plural forms are possible.

We use a hammer to drive in a nail. OR We use hammers to drive in nails.
We use an adjective to modify a noun. OR We use adjectives to modify nouns.

Mixtures of singular and plural forms are also possible.

We use a hammer to drive in nails.
We use an adjective to modify nouns. OR We use adjectives to modify a noun.
Subjects should agree with their verb. OR Subjects should agree with their verbs.