Learn Hardcore Japanese: iyahon ga kowareta node, atarasii iyahon wo kaimasu. - My earphones broke, so I will buy new ones.

が after イヤホン marks the subject of the intransitive verb 壊れた (broke).
- イヤホンが壊れた = The earphones broke.
を before 買います marks the direct object of the verb 買います (to buy).
- 新しいイヤホンを買います = I will buy new earphones.

So:

[イヤホン] が壊れた – The earphones are what broke.
新しい [イヤホン] を買います – New earphones are what I will buy.

Why is there no word for I in the sentence?

Japanese often omits the subject when it is clear from context.

English requires something like I: Because my earphones broke, I will buy new earphones.
Japanese can just say イヤホンが壊れたので、新しいイヤホンを買います。 and the listener understands (I) from context.

You could add it if needed:

私はイヤホンが壊れたので、新しいイヤホンを買います。
But in natural conversation, 私 is usually left out unless you need to contrast or emphasize who is doing the action.

What exactly is 壊れた, and what is its dictionary form?

壊れた (こわれた) is the plain past tense (た-form) of the verb 壊れる (こわれる).

Dictionary form: 壊れる – to break / to get broken (intransitive).
Past plain: 壊れた – broke / got broken.
Polite present: 壊れます.
Polite past: 壊れました.

So:

イヤホンが壊れた = The earphones broke.
A more polite version: イヤホンが壊れました。

Why is 壊れた (past) but 買います (non-past)? Why do the tenses not match?

Two different times are being described:

壊れた – past: the breaking already happened.
買います – non-past: the buying is a present/future intention.

Japanese non-past (ます / dictionary form) usually covers:

present: I buy… / I often buy…
future: I will buy… / I’m going to buy…

So the meaning is:

Because my earphones *broke (already), I will buy new earphones (from now / in the near future).*

Why is 壊れたので used instead of 壊れたから? What is the difference between ので and から?

Both ので and から can mean because.

A ので B – often sounds a bit softer, more formal, or more objective.
A から B – often sounds more direct, casual, or personal.

In this sentence:

イヤホンが壊れたので、新しいイヤホンを買います。
Sounds polite and explanatory: Since my earphones broke, I will buy new ones.

You could also say:

イヤホンが壊れたから、新しいイヤホンを買います。
This is perfectly natural, just a bit more casual/direct.

Grammatically, both are correct.

How do I form ので grammatically? What comes before ので?

You attach ので to the plain form of verbs and adjectives:

Verb (plain):
壊れたので – because (they) broke
忙しいので – because (I’m) busy
い-adjective:
寒いので – because it’s cold
な-adjective / noun: add な before ので:
便利なので – because it’s convenient
学生なので – because (I’m) a student

In your sentence:

壊れた + ので → 壊れたので

Why do we repeat イヤホン instead of saying something like them?

Japanese does not use pronouns (it, them) as much as English does. It often:

Repeats the noun, or
Drops it if it’s obvious, or
Uses の to replace the noun.

So your original:

イヤホンが壊れたので、新しいイヤホンを買います。
literally: Because earphones broke, (I) will buy new earphones.

A very natural alternative is:

イヤホンが壊れたので、新しいのを買います。
Here の stands in for イヤホン and means a new one / new ones.

All of these are natural; Japanese does not need a word like them here.

Can I say 新しいイヤホンを買います in a different word order, like イヤホンを新しい買います?

You cannot say イヤホンを新しい買います. In Japanese:

Adjectives like 新しい must come directly before the noun they modify:
- 新しいイヤホン = new earphones
- Not イヤホン新しい (before a verb) in this kind of sentence.

The basic order is:

[adjective] + [noun] + を + [verb]
新しいイヤホンを買います。 – I will buy new earphones.

Japanese word order is generally:

[reason clause], [object] を [verb].
So your sentence is already in the natural order.

Is イヤホン singular or plural? Does it mean earphone or earphones?

Japanese nouns usually do not mark singular vs. plural.

イヤホン can mean:
- an earphone, an earpiece
- earphones, a pair of earphones
- depending on context.

In this sentence:

イヤホンが壊れたので、新しいイヤホンを買います。
would normally be understood as:
- Because my earphones (the pair) broke, I will buy new earphones.

What is the difference between 壊れる and 壊す?

They are a typical intransitive / transitive pair:

壊れる (intransitive) – to break / to get broken
- Focus on the thing that breaks; no direct agent.
- イヤホンが壊れた。 – The earphones broke.
壊す (transitive) – to break (something)
- Someone actively breaks something.
- 弟がイヤホンを壊した。 – My younger brother broke the earphones.

In your sentence the focus is on the result (they ended up broken), so 壊れた is used.

Why is it イヤホンが壊れた and not イヤホンは壊れた? Can I use は here?

Both が and は are possible, but they have different nuances:

が marks the subject and often introduces new or neutral information:
- イヤホンが壊れたので… – The earphones (the ones that broke) → reason.
は marks the topic and contrasts or emphasizes:
- イヤホンは壊れたので、新しいイヤホンを買います。
  Could sound like:
  - As for the earphones, since they broke, I’ll buy new ones (maybe in contrast with something else).

In a simple “because they broke” explanation, が is the default and sounds very natural.

Can I drop the particles が or を in casual speech?

In casual spoken Japanese, particles が and を are often dropped, especially when the meaning is still clear:

イヤホン、壊れたので、新しいイヤホン買います。
イヤホン壊れたから、新しいの買う。

However:

In writing, textbooks, and more formal speech, it is better to keep が and を.
As a learner, you should first get comfortable using them correctly before copying native omissions.

Is the level of politeness consistent in this sentence?

Yes. You have:

Plain past: 壊れた
Polite non-past: 買います

This mix is actually very common when the main verb (here 買います) is in polite form. The overall sentence feels polite enough.

If you want everything polite:

イヤホンが壊れたので、新しいイヤホンを買います。 (already fine)
Or slightly more formal / regretful:
- イヤホンが壊れてしまったので、新しいイヤホンを買います。
  (Since my earphones unfortunately ended up breaking…)

What is the difference between イヤホン and ヘッドホン?

Modern usage in Japanese:

イヤホン: earphones / earbuds – small, go in or just inside the ears.
ヘッドホン: headphones – larger, go over or on the ears with a headband.

So:

If you mean tiny in-ear devices, イヤホン is correct for this sentence.

Is the comma 、 before 新しい necessary?

The 、 is optional, but common.

イヤホンが壊れたので新しいイヤホンを買います。
イヤホンが壊れたので、新しいイヤホンを買います。

Both are correct. The comma simply makes the sentence easier to read by visually separating the reason clause from the main clause.