Learn Hardcore Japanese: daigaku de ha doukyuusei to ongaku no bukatu ni hairitakatta ga, arubaito ni isogasikute hairenakatta. - At university I wanted to join a music club with my classmates, but I was too busy with my part-time job to join.

If you say only 大学で, it just states the location; with 大学では, you’re contrasting it with other times/places or setting “university life” as the overall topic of what follows.

What exactly does 同級生と mean here, and what is the role of と?

同級生 = classmates (same school year).
と here means “together with / with”, not “and” in a list.

So 同級生と音楽の部活に入りたかった literally is:

[I] wanted to join the music club with my classmates.

Without と, it would just be I wanted to join the music club, with no mention of doing it together.

Why is it 音楽の部活 instead of 音楽部?

Both can be used, but the nuance is slightly different:

音楽部 – a fixed compound noun: the music club (the official name/type of club).
音楽の部活 – a club of music / a music-related club.

音楽の部活 feels a bit more descriptive and conversational:

literally a club (部活) of music (音楽).

In many contexts, 音楽部 would also be natural, especially if that’s the official club name. Here 音楽の部活 just describes a music club in general.

Why is に used with 部活 (i.e., 部活に入る)?

In Japanese, 入る is used for joining a group/club/team, and that group takes に:

サークルに入る – join a circle/club
会社に入る – join a company
部活に入る – join a club activity

You can think of it literally as “enter into” the club, so the club is the target of entering, marked by に.

How is 入りたかった formed, and what does it imply?

Base verb: 入る – to enter / to join.

Take the ます-stem (連用形):
- 入る → 入り
Add たい to express desire:
- 入りたい – want to join
Put たい in the past tense:
- 入りたかった – wanted to join

So 入りたかった = I wanted to join.
Grammatically, たい behaves like an い-adjective:

入りたくない – don’t want to join
入りたくなかった – didn’t want to join

The subject is usually I (the speaker) unless context says otherwise.

What is the function of が between 入りたかった and アルバイトに忙しくて?

Here が is a conjunction meaning “but / although”:

入りたかったが、アルバイトに忙しくて…
→ I wanted to join, but I was busy with my part-time job, so…

Compare:

が / けれど(も) – conjunctions meaning but/although, slightly more formal than けど.
でも – sentence-initial but / however.
のに – even though / despite with a stronger sense of frustration or contradiction.

Using が here is fairly neutral and fits written or semi-formal style.

What does アルバイトに忙しくて mean exactly, and why に?

忙しい = busy.
The pattern N に忙しい means “busy with N”:

仕事に忙しい – busy with work
勉強に忙しい – busy with studying
アルバイトに忙しい – busy with a part-time job

So アルバイトに忙しくて = being busy with my part-time job and… / because I was busy with my part-time job…

You might also hear アルバイトで忙しくて.

に忙しい focuses on what occupies you.
で忙しい sounds more like “busy due to/at X,” but they often overlap in everyday speech.

What is the role of the て-form in 忙しくて入れなかった?

The て-form here connects two clauses and shows a reason or cause:

アルバイトに忙しくて、入れなかった。
→ I was busy with my part-time job, so I couldn’t join.

Patterns:

Adj (い) → くて: 忙しい → 忙しくて
Clause A て、Clause B: A happens and so B / B because A.

So 忙しくて can be read as “because (I was) busy”.

Why is it 入れなかった and not 入らなかった?

入れなかった is the potential form in the negative past:

入る → 入れる (potential: can join / be able to join)
入れる → 入れない – cannot join
入れない → 入れなかった – could not join

So 入れなかった specifically means “couldn’t join (was not able to)”, usually because of some obstacle (here: being busy with work).

If you said 入らなかった, it would simply mean “didn’t join”, which could be by choice, not necessarily because you couldn’t.

The original sentence emphasizes inability, so 入れなかった is more natural.

Could this sentence use のに instead of が? What would change?

Yes, you could say:

大学では同級生と音楽の部活に入りたかったのに、アルバイトに忙しくて入れなかった。

のに also connects two clauses, but it adds a stronger sense of “despite / even though / it’s unfortunate that…”.

Nuance:

が – neutral “but”.
のに – “even though I wanted to…, (sadly) I couldn’t.”

Using のに highlights the speaker’s frustration or regret more strongly than が.

Is it okay to say 大学では同級生と音楽の部活に入りたかったですが…?

Grammatically it’s possible, but sounds a bit off in style in most natural contexts.

〜たかったが、〜 is fine in written / formal style.
〜たかったですけど、〜 is fine in polite spoken Japanese.

Mixing plain past (入りたかった) with です／ます on が (ですが) can sound stylistically inconsistent unless you’re very deliberately blending styles.

More natural alternatives:

大学では同級生と音楽の部活に入りたかったけど、アルバイトに忙しくて入れなかった。 (casual)
大学では同級生と音楽の部活に入りたかったのですが、アルバイトに忙しくて入れませんでした。 (polite)

Can the word order be changed, like putting アルバイトに忙しくて earlier?

Yes, Japanese word order is relatively flexible as long as clause relationships stay clear. For example:

アルバイトに忙しくて、大学では同級生と音楽の部活に入りたかったが、入れなかった。

This is grammatical, but the focus slightly changes.
The original:

大学では…入りたかったが、アルバイトに忙しくて入れなかった。

First sets “at university” and “wanting to join the club with classmates” as the main situation, then gives the reason.

Reordering is possible, but the original flows and focuses more naturally on university life as the main topic.

Who is the subject in this sentence? Why is I not explicitly mentioned?

The subject (I) is omitted, which is very common in Japanese.

Full version could be:
私は大学では同級生と音楽の部活に入りたかったが、アルバイトに忙しくて入れなかった。

Because context usually makes it obvious who’s talking about their own experiences, Japanese normally drops 私 (I).

By default, if no subject is stated and the sentence is about personal desires (〜たい) and experiences, it is assumed to be the speaker.