hensinsuru mae ni, kenmei to sitagaki wo mou itido kakuninsita.

With 前に, Japanese normally uses:

• the dictionary form of a verb if the action has not happened yet at that point
• a noun directly before 前に

So 返信する前に means before replying.

The logic is:

• first: check the subject line and draft
• later: send the reply

Because the reply has not happened yet at the time of checking, する is used, not した.

前に means before.

In this sentence, it connects two actions:

• 返信する = to reply
• 確認した = checked

So the structure is:

• 返信する前に = before replying
• 件名と下書きをもう一度確認した = checked the subject line and draft one more time

A very common pattern is:

• Verb (dictionary form) + 前に = before doing something

Examples:

• 食べる前に手を洗う = Wash your hands before eating.
• 寝る前に本を読む = Read a book before sleeping.

The に marks the time point before X as the point at which the main action happens.

So in 返信する前に, the に helps mean:

• at the time before replying
• more naturally in English: before replying

Sometimes learners want to think of 前 by itself as just front/before, but 前に is the normal expression in this grammar pattern.

Yes, 返信前に is also possible.

Compare:

• 返信する前に = before replying
• 返信前に = before the reply / before replying

The version with する sounds a little more explicitly verbal, because it clearly shows the action to reply.
The noun version 返信前に is shorter and also natural, especially in practical or written contexts.

In this sentence, 返信する前に is very standard and easy to understand.

Because they are both direct objects of 確認した.

Japanese often links multiple nouns with と and puts one particle after the whole list:

• 件名と下書きを確認した
• checked the subject line and the draft

So the structure is:

• 件名 = subject line
• と = and
• 下書き = draft
• を = marks the whole combined object
• 確認した = checked

This is completely normal.
You do not need:

• 件名を下書きを確認した ❌

In this context:

• 件名 = the subject line of an email or message
• 下書き = a draft

So this sentence is talking about checking the email’s:

• subject line
• draft content

A learner might know 件 from counters, but in 件名, the whole word means subject/title line, especially for emails.

And 下書き often means:

• a rough draft
• a saved draft
• a preliminary version of writing

もう一度 means once more, again, or one more time.

• 一度 = once
• もう一度 = once again

So:

• 確認した = checked
• もう一度確認した = checked again / checked one more time

Here it suggests the speaker had probably already looked at the subject line and draft before, and checked them again before sending the reply.

Because the sentence is describing a completed action in the past:

• 確認した = checked

The whole sentence is narrating what someone did:

• before replying, they checked the subject line and draft again

Japanese often puts the main action of the sentence at the end, and here that main action is in past tense.

If it were non-past:

• 確認する = check / will check

Then it could describe a habit or future action depending on context.

Yes, the subject is omitted.

Japanese very often leaves out the subject when it is understood from context. So this sentence does not explicitly say:

• I checked
• she checked
• we checked

Any of those could be possible depending on the surrounding context.

In many textbook-style examples, English speakers often interpret it as:

• I checked the subject line and draft again before replying

But Japanese itself does not force that interpretation unless more context is given.

Japanese often puts background information first and the main action last.

So the sentence is structured like this:

• 返信する前に = before replying
• 件名と下書きを = the subject line and draft
• もう一度 = once more
• 確認した = checked

This is very natural Japanese word order:

1. time/condition
2. objects or other details
3. main verb at the end

English usually says:

• Before replying, I checked the subject line and draft again.

So the meaning matches, even though the order feels different.

確認する is a suru-verb.

That means:

• 確認 is a noun meaning confirmation/check
• adding する turns it into the verb to confirm / to check

Common forms:

• 確認する = to check
• 確認します = check / will check (polite)
• 確認した = checked
• 確認して = check and...

This pattern is extremely common in Japanese, especially with words of Sino-Japanese origin:

• 勉強する = to study
• 連絡する = to contact
• 返信する = to reply

So both 返信する and 確認する in this sentence are suru-verbs.