Learn Hardcore Latin: Avus pectinem suum iterum amisit et nunc eum in cubiculo quaerit. - Grandfather has lost his comb again and now looks for it in the bedroom.

Because avus is in the nominative singular, the case typically used for the subject of a finite verb in Latin. Here, avus means grandfather, and he is the one doing both actions: amisit (lost) and quaerit (is looking for).

Why is pectinem spelled that way instead of pecten?

Pecten is the dictionary form, but here it is the direct object of amisit, so it must be in the accusative singular: pectinem.

nominative: pecten = comb
accusative: pectinem = comb as the thing being lost or sought

Latin changes noun endings depending on their role in the sentence.

What does suum mean here, and why is it not eius?

Suum means his own. It refers back to the subject of the clause, which is avus.

So:

pectinem suum = his own comb

Latin uses suus, sua, suum when the possessor is the subject of the clause.

By contrast, eius would usually mean his or her referring to someone else, not the subject. So would sound more like belonging to another person.