oneul achime imeire paireul cheombuhaneun geol kkamppakhaesseo.

Here, 오늘 아침에 means this morning.

The particle 에 is often used with time expressions to mean at / on / in. So:

• 오늘 아침에 = this morning
• 3시에 = at 3 o’clock
• 월요일에 = on Monday

A useful note: with words like 오늘, 내일, and 어제, 에 is usually not used by itself. But with longer expressions such as 오늘 아침, 오늘 저녁, or 어제 밤, using 에 is very common and natural.

This 에 is doing a different job.

In 이메일에 파일을 첨부하다, 이메일에 means to the email or in the email. It marks the target that the file is being attached to.

So the structure is:

• 이메일에 = to the email
• 파일을 = the file
• 첨부하다 = to attach

In other words, Korean treats the email as the place/target where the file gets attached.

Because 파일 is the direct object of 첨부하다.

You are attaching the file, so 파일 gets the object particle:

• 파일을 첨부하다 = to attach a file

So in this sentence:

• 이메일에 = to the email
• 파일을 = the file
• 첨부하는 걸 = attaching it / the act of attaching it

Literally, 첨부하는 걸 is a shortened spoken form of 첨부하는 것을.

Breakdown:

• 첨부하다 = to attach
• 첨부하는 = attaching / that one attaches
• 것 = thing
• 것을 = the thing (object form)
• 걸 = casual contraction of 것을

So 첨부하는 걸 깜빡했어 literally means something like:

• I forgot the act of attaching it
• more naturally: I forgot to attach it

This is a very common Korean pattern:

• V-는 것을 깜빡하다
• V-는 걸 잊어버리다

It means to forget to do something.

This is an important nuance.

• 첨부하는 걸 깜빡했어 = I forgot to attach it
• 첨부한 걸 깜빡했어 = more like I forgot that I attached it

So:

• -는 것 often points to an action you were supposed to do
• -ㄴ/은 것 often points to a completed action or fact

In this sentence, the speaker did not attach the file and forgot to do it, so 첨부하는 걸 is the natural choice.

걸 here is not a separate dictionary word. It is a contraction of 것을.

So:

• 것을 → 걸 in casual speech

Examples:

• 먹는 것을 잊었어 → 먹는 걸 잊었어
• 가는 것을 깜빡했어 → 가는 걸 깜빡했어

This shortening is extremely common in spoken Korean and informal writing.

Important: this 걸 is not the same as the sentence-ending -걸 you may see elsewhere.

깜빡하다 means to forget absentmindedly, often in a casual, everyday way.

So 깜빡했어 has a nuance like:

• I totally forgot
• It slipped my mind
• I forgot without meaning to

Compared with 잊어버렸어:

• 깜빡했어 = more casual, often for small mistakes or momentary forgetfulness
• 잊어버렸어 = more general forgot, and can sound a bit more neutral or broader

In this sentence, 깜빡했어 is very natural because forgetting to attach a file is exactly the kind of everyday mistake this verb is often used for.

했어 is the casual/informal past tense form.

The base verb is:

• 깜빡하다 = to forget absentmindedly

Past tense:

• 깜빡했어 = forgot

This ending is used with friends, family, or people you speak casually with.

More polite versions would be:

• 오늘 아침에 이메일에 파일을 첨부하는 걸 깜빡했어요.
• even more formal depending on context: 오늘 아침에 이메일에 파일을 첨부하는 것을 깜빡했습니다.

So the original sentence sounds natural in everyday conversation.

The word order is natural, but Korean is more flexible than English because particles show each word’s role.

This sentence is organized like this:

• 오늘 아침에 = this morning
• 이메일에 = to the email
• 파일을 = the file
• 첨부하는 걸 = attaching it
• 깜빡했어 = forgot

So the overall structure is basically:

This morning, to the email, the file, attaching it, I forgot.

That sounds strange in English, but in Korean it works because the main verb comes at the end and the particles make the relationships clear.

Yes. A few natural alternatives are:

• 오늘 아침에 이메일에 파일 첨부하는 걸 깜빡했어.
• 오늘 아침에 이메일에 파일을 첨부하는 걸 잊어버렸어.
• 오늘 아침에 이메일 보낼 때 파일 첨부하는 걸 깜빡했어.

These all express roughly the same idea, though the nuance changes a little.

The original sentence is already very natural and useful, especially for spoken Korean.