balpyohal ttae silsuhalkka bwa memojange jungyohan pyohyeoneul jeogeo dwoyo.

때 means time or when. After a verb, -(으)ㄹ 때 means when doing or at the time of doing something.

So:

• 발표하다 = to give a presentation
• 발표할 때 = when giving a presentation / when I present

Because 발표하다 ends in a vowel sound before 하다, it becomes 발표할 때 rather than 발표할을 때.

In this sentence, it refers to the situation during a presentation or when I have to present.

-(으)ㄹ까 봐 is a very common grammar pattern meaning:

• because I’m worried that...
• for fear that...
• in case...

So 실수할까 봐 means:

• because I’m worried I might make a mistake
• for fear that I’ll make a mistake

It expresses concern or anxiety about a possible situation.

Not in a literal sense.

In -ㄹ까 봐, learners often notice 봐 and think it means see, but in this expression the whole chunk works as a set grammar pattern expressing worry/concern.

So it is better to learn 실수할까 봐 as one unit meaning:

• because I’m afraid I might make a mistake

rather than trying to interpret it word by word as see whether I make a mistake.

Because -(으)ㄹ까 봐 attaches to the verb stem, not to a conjugated form.

Here is the breakdown:

• dictionary form: 실수하다
• verb stem: 실수하-
• add -ㄹ까 봐
• result: 실수할까 봐

So you do not say 실수해까 봐.

This is the normal pattern:

• 가다 → 갈까 봐
• 늦다 → 늦을까 봐
• 실수하다 → 실수할까 봐

Because 메모장 is the place/surface/location where the writing goes, so it takes 에.

With 적다 (to write down), Korean often uses this pattern:

• A에 B를 적다
• write B in/on A

So in this sentence:

• 메모장에 = in/on a memo pad or memo app
• 중요한 표현을 = important expressions
• 적어 둬요 = write down and keep there

The object marker 을/를 goes with the thing being written, not the notebook/app itself.

This comes from 적어 두다.

• 적다 = to write down
• -아/어 두다 = do something in advance and leave it that way for later

So 적어 두다 means:

• to write down and keep it there
• to write down beforehand
• to make a note in preparation

In this sentence, that nuance is important. The speaker is not just writing randomly; they are preparing ahead of time so the notes will be available when needed.

적어 둬요 is the polite present form, and it often sounds natural in English as I write it down beforehand or I keep it written down.

적어요 simply means I write it down.

적어 둬요 adds the idea of:

• doing it in advance
• doing it for later use
• leaving it ready/prepared

So:

• 중요한 표현을 적어요 = I write down important expressions
• 중요한 표현을 적어 둬요 = I write down important expressions beforehand so I have them ready

That extra preparation nuance is exactly why 적어 두다 is used here.

It can be either, depending on context.

메모장 literally means something like:

• memo pad
• notepad
• notes app / memo app

In everyday Korean, it can refer to a physical notepad, but very often it also means the memo app on a phone or computer. This sentence works naturally for either one.

표현 means expression, so it is often broader than a single word.

It can refer to:

• useful phrases
• ways of saying something
• key wording for a presentation

So 중요한 표현 suggests the speaker is noting down important phrases or expressions they may want to use during the presentation, not just isolated vocabulary items.

The subject is omitted because Korean often leaves out subjects when they are understood from context.

Here, the implied subject is most naturally I because:

• the sentence talks about the speaker’s worry
• the speaker is the one writing notes
• the ending -요 shows polite speech, not the subject

So the full idea could be:

• 저는 발표할 때 실수할까 봐 메모장에 중요한 표현을 적어 둬요.

But saying 저는 is not necessary unless you want to emphasize it.

This is standard Korean spacing.

• 실수할까 봐
• 적어 둬요

In both cases, the second part is treated as a separate auxiliary element in standard writing, so a space is used.

In speech, they sound very closely connected, so learners sometimes expect them to be written as one unit. But the standard written forms are:

• -ㄹ까 봐
• -아/어 두다

So the spacing in this sentence is normal and correct.