koohii ga sameru mae ni, meeru no henzi wo kaite simau.

Because コーヒー is the thing that does the verb 冷める.

• 冷める means to get cold / to cool down and is an intransitive verb.
• So the coffee is the subject of that verb, which is why が is natural.

Here, コーヒーが冷める前に means before the coffee gets cold.

Could you use は instead? Sometimes, yes, but it would change the nuance. は would sound more like you are making coffee the topic or contrasting it with something else. In this sentence, が is the most straightforward choice.

Because with 前に, verbs usually appear in the plain non-past form when they describe an event that has not happened yet at that point.

Pattern:

• Verb dictionary form + 前に = before doing / before something happens

So:

• 冷める前に = before it gets cold
• 行く前に = before going
• 寝る前に = before sleeping

Even though English often uses a past-looking idea in translation, Japanese uses the dictionary form here.

前に means before.

It connects the action in the main part of the sentence with something that has not happened yet.

So the structure is:

• コーヒーが冷める前に = before the coffee gets cold
• メールの返事を書いてしまう = write the email reply completely / get it written

Together, it means that the writing should happen earlier than the coffee getting cold.

Because 前 is a noun, and に marks the time point relative to which something happens.

Think of it like this:

• 前 = before / the front / prior point
• 前に = at the time before ...

This is a very common pattern:

• ご飯の前に = before the meal
• 日本へ行く前に = before going to Japan

So 前に works almost like a fixed expression meaning before.

This is the classic intransitive vs transitive pair.

• 冷める = something cools down
• 冷ます = someone cools something down

Examples:

• コーヒーが冷める。 = The coffee gets cold.
• コーヒーを冷ます。 = Cool the coffee down.

In your sentence, the coffee cools down by itself, so 冷める is the correct choice.

Because の connects the two nouns and shows their relationship.

• メール = email
• 返事 = reply / response
• メールの返事 = a reply to the email

It is similar to saying:

• 手紙の返事 = a reply to a letter
• 質問の答え = the answer to a question

So メールの返事 is a natural noun phrase meaning the email reply or a reply to the email.

Because 返事 is the direct object of 書く.

Here the verb is not する. The verb is 書く, so what is being written?

• 返事を書く = write a reply

That is why を is used.

Compare:

• 返事をする = give a reply / respond
• 返事を書く = write a reply

Both are possible, but this sentence specifically focuses on writing the reply.

〜てしまう adds a nuance of completing something fully or getting it done.

So:

• 書く = write
• 書いてしまう = write it completely / get it written / finish writing it

In this sentence, it gives a feeling like:

• I’ll go ahead and finish writing the reply before the coffee gets cold.

Sometimes 〜てしまう can also express regret, like oops, I ended up doing it, but that is probably not the main nuance here. Here it sounds more like completion or doing it before it is too late.

Yes, 〜てしまう often has two common meanings:

1. completion
   • finish doing something
2. regret / unintended result
   • end up doing something unfortunately

Examples:

• 宿題をやってしまった。 = I finished my homework.
• 財布をなくしてしまった。 = I ended up losing my wallet.

In your sentence, the context suggests the completion meaning, not regret. It sounds like the speaker wants to get the reply written before the coffee cools down.

The subject is omitted because it is understood from context. This is very common in Japanese.

In English, you usually need to say I, you, he, and so on. In Japanese, if it is obvious, it is often left out.

So this sentence could imply:

• I will write the reply before the coffee gets cold.
• Or, depending on context, someone else will.

Without more context, the natural assumption is often the speaker.

Because the sentence is written in the plain style.

Japanese examples are often shown in plain form because it is the base form used in grammar explanations and casual speech.

You could make it polite:

• コーヒーが冷める前に、メールの返事を書いてしまいます。

That would be more suitable in polite conversation. The meaning is basically the same, but the tone is different.

No, it is not strictly necessary, but it helps readability.

The comma marks a pause after the before clause:

• コーヒーが冷める前に、メールの返事を書いてしまう。

Without the comma, the sentence is still correct:

• コーヒーが冷める前にメールの返事を書いてしまう。

Japanese punctuation is often more flexible than English punctuation.

Yes, that is also natural.

Compare:

• メールの返事を書く
• メールに返事を書く

Both mean something like write a reply to the email.

The difference is small:

• メールの返事 treats reply to the email as one noun phrase.
• メールに返事を書く makes the email the target marked by に.

Both are good Japanese. The sentence you have is perfectly natural.