Learn Hardcore Korean: keun soriga gongbureul banghaehaeyo.

It marks 큰 소리 (loud noise) as the grammatical subject. You use -가 after a noun ending in a vowel sound (like 소리). If the noun ended in a consonant, you’d use -이. Using the subject particle here presents “a loud noise” as the thing doing the action (interfering).

Can I use -는 instead: 큰 소리는 공부를 방해해요?

Yes. -는/은 marks a topic. With 큰 소리는, you sound like you’re making a general statement or setting up a contrast: “As for loud noise(s), (it) interferes with studying.” With -가, you’re simply stating that a (or the) loud noise is interfering, often as new information.

Why is it 공부를 and not 공부에?

The verb 방해하다 (“to disturb/interfere with”) takes a direct object, so you mark it with -를/을: 공부를 방해하다. If you switch to the expression 방해가 되다 (“to be an interference”), you use -에: 공부에 방해가 돼요.

What is 방해해요 made from?

It’s the polite present form of the verb 방해하다 (noun 방해 + verb 하다). Conjugation: 방해하다 → 방해해요 (polite), 방해합니다 (formal), 방해해 (casual). The base/dictionary form is 방해하다.

Does 큰 mean “big”? Are we talking about size?