Learn Arabic (Modern Standard): عندنا أفلام قديمة.

أفلام is a broken plural. Unlike regular plurals that just add an ending, broken plurals change the internal spelling of the root word itself. The singular is فيلم (film), and it breaks its original shape to become أفلام.

Is there a rule or pattern to help me remember how to make this plural?

Yes! While broken plurals must be memorized, they follow specific rhythmic patterns. أفلام follows a very common pattern that sounds like af-aal. You will hear this exact same rhythm in other A1 words, like how قلم (pen) becomes أقلام (pens) or ولد (boy) becomes أولاد (boys).

Why is the adjective for old feminine singular when films is a plural word?

This is one of the most important rules in Arabic! All non-human plurals (things, animals, concepts) are treated as grammatically feminine singular. Since films are objects, the adjective describing them must take the feminine singular form قديمة rather than a plural form.

I learned that Arabic doesn't have a verb for to have. How does the first word work here?

You are correct! To say to have, Arabic uses a preposition with a pronoun suffix attached to the end. The word عندنا is made of عند (at or with) plus the suffix نا (us). Literally, it translates to at us, which is the standard way Arabic expresses that we have something.