ijeobeorilkka bwa saeroun daneoneun gongchaege memohae dwoyo.

-ㄹ까 봐 expresses worry, concern, or precaution about something that might happen. It often corresponds to ideas like:

• for fear that ...
• because I might ...
• in case ...

So 잊어버릴까 봐 means something like because I might forget or for fear of forgetting.

A useful way to think about it:

• possible negative outcome: forgetting
• preventive action: writing the word down

That pattern is very common in Korean.

Both can mean to forget, but 잊어버리다 often sounds more like to forget completely or to end up forgetting, with a stronger sense that the action is finished or unfortunate.

Compare:

• 잊다 = to forget
• 잊어버리다 = to forget completely / to end up forgetting

In everyday Korean, 잊어버리다 is very common because it adds a natural, conversational nuance. Here it emphasizes the speaker’s concern about fully forgetting the new word.

Originally, 버리다 means to throw away, but when attached to another verb, it often adds a nuance like:

• doing something completely
• finishing it
• having an unfortunate or regrettable result

So in 잊어버리다, it does not literally mean throw away. It makes the forgetting sound more complete or more like an unwanted result.

This use of -아/어 버리다 is very common in Korean.

Here, 는 marks 새로운 단어 as the topic: the thing the speaker is talking about in general.

So the sentence is structured more like:

• As for new words, I write them down in a notebook in case I forget them.

Using 는 gives a general, habitual feeling:

• When it comes to new words, I do this.

If you said 새로운 단어를, it would focus more directly on the object of the verb, and the sentence would feel a bit less like a general statement about the speaker’s habit.

에 marks the location/place where something is written.

So:

• 공책에 메모하다 = to make a note in a notebook

Here 에 works like in or on, depending on natural English translation.

You often use 에 with verbs of writing or putting something somewhere:

• 종이에 쓰다 = write on paper
• 노트에 적다 = write in a notebook
• 벽에 붙이다 = stick onto the wall

This is an important point.

• 메모하다 = to make a note
• 메모해 두다 = to make a note and leave it there for later use

The grammar -아/어 두다 means do something in advance or do something and keep it that way.

So 메모해 둬요 does not just mean I write it down. It means something closer to:

• I write it down beforehand
• I make a note of it so I’ll have it later
• I keep a written note

That fits perfectly with 잊어버릴까 봐: the speaker is taking a precaution.

둬요 is the contracted spoken form of 두어요.

The base verb is:

• 두다

With polite present ending:

• 두어요

In natural speech and writing, this usually contracts to:

• 둬요

So:

• 메모해 두어요 → 메모해 둬요

Both are correct, but 둬요 is much more common in everyday Korean.

It is:

• 메모하다 → 메모해
• 두다 → 둬요

Combined:

• 메모해 두다 → 메모해 둬요

This is a verb + 아/어 두다 structure. It means:

• do the action now
• keep the result for later

Other examples:

• 문을 열어 둬요 = I leave the door open
• 미리 준비해 둬요 = I prepare it in advance
• 전화번호를 적어 둬요 = I write down the phone number for later

A simple breakdown is:

• 잊어버릴까 봐 = because I might forget / in case I forget
• 새로운 단어는 = as for new words
• 공책에 = in a notebook
• 메모해 둬요 = I write them down and keep the note

So the logic is:

1. possible problem: forgetting
2. topic: new words
3. place: notebook
4. preventive action: note them down

Korean often places the reason or concern first, and the main action later, which is exactly what happens here.

Yes. Korean very often leaves out the subject when it is obvious from context.

Here, the implied subject is probably:

• I

So the full idea is something like:

• I write new words down in a notebook in case I forget them.

This omission is completely normal in Korean and happens constantly in everyday speech.

It is in the informal polite style, often called 해요체.

The ending -아요/어요 makes it polite but not formal-stiff. It is appropriate for:

• everyday conversation
• speaking politely to someone you do not need to be highly formal with
• many textbooks and general explanations

If you wanted a more formal style, you could say:

• 메모해 둡니다

If you wanted casual speech:

• 메모해 둬

Yes, in many situations -아/어 놓다 can be similar to -아/어 두다.

• 메모해 둬요
• 메모해 놔요

Both can mean that you make a note and keep it for later. However:

• -아/어 두다 often emphasizes preparation or doing something in advance
• -아/어 놓다 often emphasizes the resulting state being left that way

In real conversation, the two are often close in meaning, and both are common. In this sentence, 메모해 둬요 sounds very natural because the speaker is doing it as a precaution.

It most naturally sounds like new words in general, as a habit.

That feeling comes mainly from:

• 새로운 단어는 with 는
• the present polite ending 메모해 둬요, which often describes a regular practice

So the sense is:

• When I come across new words, I write them down in a notebook so I won’t forget them.

Even though 단어 is singular in form, Korean often uses a bare noun like this to refer to things in general, not just one specific item.