Learn Hardcore Japanese: neru mae ni ongaku wo kiku to, nemuri ga hukaku narimasu. - If you listen to music before going to sleep, your sleep becomes deeper.

寝る前に literally means before sleeping. In Japanese, you attach 前に to the plain (dictionary) form of a verb to indicate that the main action happens before that verb. The particle に marks 前 as a time expression – telling you when the following action (listening to music) takes place.

Why is 音楽を聞くと translated as “if/when you listen to music”? What role does と play here?

The particle と in this context is a conditional that expresses a natural, habitual, or inevitable result. In the pattern AとB, whenever A happens, B follows as a general truth. Here, 音楽を聞くと means “if/when you listen to music”, and the result clause 眠りが深くなります (“sleep becomes deeper”) describes what generally follows.

Why is 深く used instead of 深い in 深くなります?

To modify a verb with an い-adjective, you convert the adjective into an adverb by dropping い and adding く. Thus 深い (deep) → 深く (deeply). Then 深くなる literally means “become deep” (i.e. “grow deeper”).

Why is 眠り followed by が in 眠りが深くなります? Could you use は instead?

The particle が marks the grammatical subject or the focus of a neutral, descriptive statement. Here, 眠りが深くなる states “sleep becomes deeper” as a fact. You could use は if you wanted to contrast or topicalize sleep (e.g. 眠りは深くなりますが…), but in general factual statements, が is preferred to introduce new information.

What’s the difference between 聞く and 聴く in 音楽を聞く?

Both 聞く and 聴く can mean “to listen,” but:

聞く is more general (“to hear” or “to listen”).
聴く emphasizes intentional, attentive listening (often used for music or lectures).
In everyday usage, learners and many native speakers simply use 聞く for listening to music.

Could I use 聞いたら or 聞けば instead of 聞くと, and what difference would that make?

Yes. You could say 寝る前に音楽を聞いたら眠りが深くなります or 聞けば. The differences:

と implies a natural, automatic result for habitual actions or general truths.
たら is used for “when/if” in both hypothetical and past contexts and often feels more flexible or casual.
ば is a more formal/書き言葉 conditional meaning “if.”
All three can convey a similar meaning, but と stresses a cause-and-effect you can expect every time.

Why is the verb in the first clause in plain form (聞く), but the second clause uses polite form (なります)?

In the と-conditional, the protasis (the “if” clause) must use the plain (dictionary) form of the verb. The apodosis (the result clause) can be either plain or polite, depending on the desired register. Here, the speaker chose a polite tone with なります.

What does 眠り mean, and how’s it different from 眠る or 眠い?

眠り is the noun “sleep” (the state or quality of sleeping).
眠る is the verb “to sleep.”
眠い is the adjective “sleepy” (feeling drowsy).
This sentence discusses the depth of your sleep (眠り), not the act of sleeping or the feeling of sleepiness.