Learn Hardcore Latin: Puella annulum retinet, donec mater domum redeat. - The girl keeps the ring until mother returns home.

Breakdown of Puella annulum retinet, donec mater domum redeat.

Why is puella in the nominative case?

Because puella is the subject of retinet. It is the one doing the action: the girl keeps/holds onto the ring.

In Latin, the subject is usually put in the nominative case.
So:

Why is annulum in the accusative case?

Because annulum is the direct object of retinet. It is the thing being kept or held.

Latin uses the accusative for the direct object.
So:

This shows that the sentence means the girl is keeping the ring, not that the ring is doing something.

What does retinet mean exactly?

Retinet comes from retinere, meaning to hold back, keep, retain, keep possession of.

Here it is:

So retinet means , , or , depending on how naturally you want to translate it in English.