Learn Hardcore Japanese: iyahon wo wasureta node, densya de ongaku wo kikemasen. - Because I forgot my earphones, I can’t listen to music on the train.

イヤホンを忘れた
→ 忘れた (forgot) is a transitive verb. The thing you forgot (earphones) is the direct object, so it takes を.
音楽を聞けません
→ 聞けません (cannot listen) is also transitive. The thing you (can’t) listen to (music) is the direct object, so it also takes を.

In both cases, X を [verb] = “verb X.”

What exactly is 忘れた? Why not 忘れました?

忘れた is:

the plain past form of 忘れる (to forget)
roughly “forgot” in English.

Conjugation:

Dictionary form: 忘れる (to forget)
Plain past: 忘れた (forgot)
Polite past: 忘れました (forgot – polite)

In this sentence, 忘れた appears inside a subordinate clause (イヤホンを忘れたので “because I forgot my earphones”), and Japanese very often uses plain form in such clauses even when the main clause is polite.

Polite alternative (more formal/official):
イヤホンを忘れましたので、電車で音楽を聞けません。

Why isn’t there any word for “I” in the sentence?

Japanese often omits the subject (like I, you, he, she) when it’s clear from context.

In conversation, if you are talking about your earphones and your train ride, it’s obvious the subject is “I,” so Japanese omits 私.
Mentally, you can read it as:
(私は) イヤホンを忘れたので、電車で音楽を聞けません。
“(As for me,) I forgot my earphones, so I can’t listen to music on the train.”

Adding 私は is grammatically fine but often unnecessary and can sound heavier or overly explicit in casual speech.

What does ので mean here, and how is it different from から?

ので is a conjunction meaning “because / since.”

イヤホンを忘れたので、電車で音楽を聞けません。
“Because I forgot my earphones, I can’t listen to music on the train.”

Difference from から:

から: more direct, casual, often used in conversation.
イヤホンを忘れたから、電車で音楽を聞けません。
ので: sounds softer, a bit more formal or explanatory, and is very common in polite speech and writing.

Both are correct here; choice depends on tone:

to a friend: …忘れたから、音楽を聞けない。
in a polite context: …忘れたので、音楽を聞けません。

Why is the first part plain (忘れた) but the second part polite (聞けません)? Isn’t that mixing styles?

You generally shouldn’t mix plain and polite styles within the main clauses of a sentence, but it is common and natural to:

use plain form inside subordinate clauses (before ので, から, とき, etc.)
use polite form in the main clause.

So:

Subordinate reason clause (plain): イヤホンを忘れたので
Main clause (polite): 電車で音楽を聞けません。

This pattern is very typical in polite speech and does not sound strange.

Why is it 電車で and not 電車に or something else?

で marks the place where an action happens.

電車で音楽を聞けません。
→ “I can’t listen to music on the train.”
(The action 聞く/“listen” takes place at/in the train → 電車で.)

Compare:

学校で勉強する – to study at school
家で映画を見る – to watch a movie at home

You could also say 電車の中で (“inside the train”) for extra clarity or emphasis:

電車の中で音楽を聞けません。

What does 聞けません mean exactly, and how is it different from 聞きません?

聞けません is the negative potential form of 聞く.

聞く – to hear / listen / ask
聞ける – can hear / can listen
聞けません – cannot hear / cannot listen (polite)

So 音楽を聞けません means “I can’t listen to music” (it’s impossible in this situation).

聞きません would be simple negative (polite):

音楽を聞きません – “I don’t listen to music / I won’t listen to music.”
→ sounds like a choice or habit, not a lack of ability.

So here 聞けません is correct because the problem is you forgot your earphones, so you’re unable to listen.

Does 聞く here mean “to ask” or “to listen”?

聞く can mean:

to listen / to hear
to ask (a question)

The meaning is decided by context and objects:

音楽を聞く – to listen to music
先生に質問を聞く – to ask the teacher a question

In 音楽を聞けません, the object is 音楽 (“music”), so the meaning is “to listen (to music)”.

Could I say ヘッドホン instead of イヤホン?

Yes, but there is a nuance difference:

イヤホン – earphones, earbuds (small, in-ear)
ヘッドホン – (over-ear/on-ear) headphones

So:

イヤホンを忘れたので… – I forgot my earphones/earbuds.
ヘッドホンを忘れたので… – I forgot my headphones.

Grammar is identical; only the object changes.

Can I reorder the words, like 電車でイヤホンを忘れたので? Does that change the meaning?

You can reorder, but you have to be careful about what it sounds like:

イヤホンを忘れたので、電車で音楽を聞けません。
→ Natural reading: You forgot your earphones (probably before getting on), so you can’t listen to music on the train.
電車でイヤホンを忘れたので、音楽を聞けません。
→ Natural reading: You forgot your earphones on the train (you left them there), so now (after that) you can’t listen to music.

Word order in Japanese is flexible, but moving phrases changes which word they most closely connect to, and thus can change the nuance.

Is the comma (、) after ので important? Could I leave it out?

The comma 、 is mostly about readability and rhythm, not strict grammar.

With comma: イヤホンを忘れたので、電車で音楽を聞けません。
Without comma: イヤホンを忘れたので電車で音楽を聞けません。

Both are grammatically correct. The comma just makes it easier to see the boundary between the “because” clause and the main clause, similar to English:

“Because I forgot my earphones, I can’t listen to music on the train.”

How could I say this more casually or more formally?

More casual (talking to a friend):

イヤホン忘れたから、電車で音楽聞けない。
- Drop を after イヤホン (common in speech)
- Use から instead of ので
- Use plain negative potential 聞けない

Neutral–polite (original style):

イヤホンを忘れたので、電車で音楽を聞けません。

More formal/polite (e.g., announcement style):

イヤホンを忘れましたので、電車内で音楽をお聞きになることができません。
(Very polite and wordy; you’d rarely need this level in normal conversation.)