Learn Hardcore Korean: sueop junge bae-un daneoreul itji anheuryeomyeon memohaseyo. - If you don’t want to forget the words learned in class, take notes.

-으려면 is a conditional connective ending attached to a verb stem. It means “if you intend to…” or “if you want to….” Here, it attaches to the negated verb 잊지 않다 (“not to forget”), so 잊지 않으려면 means “if you don’t want to forget…”

Why does the sentence use the double negative in 잊지 않으려면, and could I say 잊지 않으려고 하면 instead?

The “double negative” comes from 잊지 않다 (“not to forget”) plus the conditional -으려면. You can indeed rephrase it as 잊지 않으려고 하면, which uses -려고 하면 (“if you try to…”). Both are grammatical and mean roughly the same thing. The nuance is subtle:
• -으려면 emphasizes your plan or intention (“if you intend not to forget…”).
• -려고 하면 can feel a bit more like a cause-and-effect (“if you try not to forget…”).

What does 수업 중에 mean, and how is it different from 수업 때 or 수업 동안?

All three can translate as “during class,” but they carry slight nuance:

수업 중에 (“in the middle of class”) uses 중(中)
- 에 to indicate a point or period within the class.
수업 때 focuses on the time when class happens, more like “when class occurs.”
수업 동안 emphasizes the whole duration of the class (“throughout class”).

In everyday speech, 수업 중에 and 수업 동안 often overlap, but 수업 중에 is most idiomatic for “while the class is going on.”

How does 배운 단어를 function here? Why is 단어 marked with 를?

배운 단어 is a noun phrase where 배운 is the past participle of 배우다 (“to learn”), so it means “words that (you) learned.” The particle 를 marks 단어 as the direct object of the verb 잊다 (“to forget”). In the subordinate clause 수업 중에 배운 단어를 잊지 않으려면, you’re literally saying “if (you) don’t want to forget the words learned in class…”

Why is 배운 단어 used instead of 배우는 단어?

배운 단어 (learned words) uses the past participle 배운, so it refers to vocabulary you’ve already learned.
배우는 단어 (words that are being learned) would imply you’re still in the process of learning them.

Since the sentence advises you not to forget what you’ve already covered, 배운 is the appropriate form.

What does 메모하세요 mean, and what nuance does the -세요 ending add?

메모하세요 is the polite imperative of 메모하다 (“to make a memo,” “to take notes,” or “to jot down”).

메모 is a loanword from English “memo.”
-하세요 is the honorific/polite command ending, so 메모하세요 translates as “Please take notes.”

Are there alternative verbs I can use instead of 메모하다 to say “take notes”?

Yes. Common alternatives include:
• 필기하다 – more formal/academic “to take notes.”
• 기록하다 – “to record,” often used for logs or keeping records.
• 적어 두다 – “to write down and keep.”

For example: 수업 중에 배운 단어를 잊지 않으려면 필기하세요.

Can I omit the 에 in 수업 중에 and say 수업 중 배운 단어를?

Yes. When 중 directly modifies a noun clause, the 에 is often dropped in everyday usage. Both 수업 중에 배운 단어를 and 수업 중 배운 단어를 are acceptable, though adding 에 (수업 중에) is slightly more formal or clear in adverbial contexts.