Learn Hardcore Korean: ipguksimsa juri gilmyeon yeogwongwa seoryureul miri junbihae duseyo. - If the immigration queue is long, prepare your passport and documents in advance.

The ending -면 is the conditional marker in Korean. It attaches to the verb/adjective stem (here 길- from 길다 “to be long”) to mean “if” or “when”. So 줄이 길면 = “if/when the line is long.”

Why is 줄이 used instead of just 줄?

줄 is the noun “line,” and in Korean you normally attach a case particle to nouns. Here 이 is the subject particle (after a consonant).
• 줄 + 이 → 줄이 = “the line (is)….”

Why is there no explicit subject like “you” in this sentence?

Korean often omits the subject when it’s clear from context. In this instruction, it’s understood that “you” (the traveler) should prepare your passport and documents.

What does 미리 add to the sentence?

미리 means “in advance” or “ beforehand.” It emphasizes preparing before you actually need to present your documents.

What is the function of -아/어 두다 in 준비해 두세요?

The construction -아/어 두다 expresses doing something in advance and leaving it ready.
• 준비하다 = “to prepare”
• 준비해 두다 = “prepare (it) and keep it ready”

Why is 준비해 두세요 in the polite imperative form?

Because it ends in -세요, which is the standard polite command form used when speaking respectfully to someone you don’t know well (e.g., fellow passengers).

Why does the sentence use 과 between 여권 and 서류?

과 is the formal conjunctive particle meaning “and” after a consonant. You could also say:
• 여권과 서류 (formal)
• 여권하고 서류 (more casual)
• 여권이랑 서류 (casual)

Can we reorder the clauses? For example, put 준비해 두세요 first?

Korean prefers to place condition/time clauses (with -면) at the front, followed by the main clause. So 줄이 길면 naturally comes first, then 여권과 서류를 미리 준비해 두세요. Reordering it would sound awkward:
✗ 여권과 서류를 미리 준비해 두세요, 줄이 길면.