Learn Polish: Aktor czuje tremę przed spektaklem. - The actor feels stage fright before the show.

This happens because of the preposition przed (before). When you use przed to talk about time (meaning "before an event"), the noun that follows it must take the Instrumental case. Because spektakl is a masculine noun, it gets the typical Instrumental ending -em.

Why is it tremę instead of trema?

Trema is a feminine noun. In this sentence, it is the direct object of the verb czuje (he feels) — it's the thing being felt. That means it needs to be in the Accusative case. For feminine nouns ending in -a, the Accusative ending is always -ę.

Can I use trema for feeling scared in general, like when watching a horror movie?

No, trema specifically means "stage fright" or the nervous butterflies you get before a public performance, an exam, or a big presentation. For general fear (like being scared of a spider or a movie), you would use a word like strach (fear).

What is the dictionary form (infinitive) of the verb czuje?

The infinitive is czuć (to feel). It follows a common pattern where the -uć ending changes to an -uj- stem in the present tense, making "he/she/it feels" czuje.