Learn Hardcore Latin: Serva cochleari parvo lac miscet et puero dat. - The female servant mixes milk with a small spoon and gives it to the boy.

Serva is nominative singular. In Latin, the subject of a finite verb is normally in the nominative case, so serva is the one doing the actions of miscet and dat.

It comes from serva, servae, a first-declension feminine noun.

Why is cochleari parvo in the ablative?

Cochleari parvo is in the ablative singular because it expresses means or instrument: the action is done with a small spoon.

Latin often uses the ablative without a preposition for this idea. So:

cochleari parvo = with a small spoon

This is called the ablative of means or instrumental ablative.

How do I know that parvo goes with cochleari?

Because adjectives in Latin agree with the nouns they modify in case, number, and gender.

Here:

cochleari = ablative singular neuter
parvo = ablative singular neuter

So parvo matches cochleari, giving the phrase cochleari parvo = with a small spoon.