jungyohan pyohyeoneun memohae duseyo.

중요한 comes from the descriptive verb 중요하다, which means to be important.

When a descriptive verb modifies a noun, Korean uses an attributive form. For 중요하다, that form is 중요한.

• 중요하다 = to be important
• 중요한 표현 = an important expression / important expressions

So here, 중요한 is directly describing 표현.

This is a very common question. In this sentence, 은 is the topic particle, not the object particle.

• 표현을 메모해 두세요 would simply mean Please make a note of the expressions
• 표현은 메모해 두세요 gives the sense of As for the important expressions, make sure you note them down

Using 은 adds emphasis or contrast. It can suggest something like:

• at least the important expressions
• when it comes to important expressions, note them down
• important expressions, make sure you write those down

So 은 here is not random—it gives a slightly stronger, more pointed feeling.

It can be either. Korean nouns usually do not have to mark singular vs. plural unless the speaker wants to make it explicit.

So 중요한 표현은 could mean:

• an important expression
• important expressions

In natural English, this sentence is often understood as plural: important expressions.

If Korean wanted to make the plural clearer, it could say 중요한 표현들은, but that is not always necessary.

This is made from:

• 메모하다 = to make a memo / take a note / jot down
• -아/어 두다 = to do something and leave it that way, often for later use or in preparation
• -세요 = polite request / instruction

So 메모해 두세요 is not just write it down. It has the nuance of:

• Please make a note of it and keep it for later
• Be sure to jot it down
• Write it down so you’ll have it later

The -아/어 두다 part is important because it suggests doing something in advance for future benefit.

The basic difference is nuance.

• 메모하세요 = Please take a note
• 메모해 두세요 = Please write it down and keep it noted for later

So 메모해 두세요 sounds a bit more like:

• this will be useful later
• don’t forget it
• keep a record of it

That makes 메모해 두세요 very natural in a classroom or study situation.

Because 해 두세요 comes from the auxiliary construction -아/어 두다.

Breakdown:

• 메모하다
• 메모해 두다
• 메모해 두세요

In standard spacing, the auxiliary verb 두다 is often written separately after -아/어:

• 해 두다
• 써 두다
• 알아 두다

So 메모해 두세요 is the standard spaced form.

You may sometimes see tighter writing in casual contexts, but 메모해 두세요 is the safe standard form.

-세요 is a polite ending used for requests, instructions, or gentle commands.

So 메모해 두세요 is polite and appropriate in many everyday situations, such as:

• a teacher speaking to students
• instructions in a textbook
• a polite reminder

It is not casual, but it is also not especially formal. It sits in the common polite register.

Compare:

• 메모해 둬 = casual
• 메모해 두세요 = polite
• 메모해 두십시오 = more formal

Yes, but it is a polite instruction rather than a harsh command.

In English, depending on context, it could feel like:

• Please note down important expressions
• Be sure to write down important expressions
• Make a note of important expressions

Because of -세요, it sounds courteous. In Korean, this kind of sentence is very common in teaching materials and does not usually sound rude.

Yes, that is a very helpful literal understanding.

A close structural reading is:

• 중요한 표현은 = as for important expressions
• 메모해 두세요 = please note them down and keep that note

That literal reading helps explain why 은 is used. The sentence is topicalizing important expressions and telling the listener what to do with them.

No. 메모 is a loanword from English memo, and 메모하다 means to memo / to take a note.

This is very common in Korean: a borrowed noun combines with 하다 to make a verb.

Other examples:

• 공부하다 = to study
• 준비하다 = to prepare
• 체크하다 = to check

So 메모하다 is a very natural everyday verb, even though part of it comes from a loanword.