Learn Hardcore Korean: munseoreul jeojanghago naseo keompyuteoreul kkeoyo. - After saving the document, I turn off the computer.

-고 나서 means “after finishing doing X, (then) Y.” It emphasizes the completion of the first action before the second starts. Plain -고 can simply list actions in sequence without highlighting completion.

Example:
- 밥을 먹고 이를 닦아요 = I eat and (then) brush my teeth.
- 밥을 먹고 나서 이를 닦아요 = I brush my teeth after I’m done eating. (clearer completion nuance)

Is 나서 necessary? Can I just say 문서를 저장하고 컴퓨터를 꺼요?

You can. -고 alone often sounds fine for sequences. -고 나서 is preferred when you want to stress “only after fully completing the first action,” which suits step-by-step instructions or when order matters.

Are 저장하고 나서, 저장한 다음에, 저장한 후에, and 저장한 뒤에 interchangeable?

Yes, all mean “after saving,” with small nuance differences:

저장하고 나서: emphasizes completion of “save.”
저장한 다음에: very common, everyday.
저장한 후에: a bit more formal/literary.
저장한 뒤에: casual, “after/behind.” You may also see 저장 후에 (noun + 후에), which is concise and more written/formal.

Why is there a space in 저장하고 나서? Is that spelling correct?

Yes. Standard spelling writes -고 나서 with a space: 저장하고 나서. The connective ending -고 attaches to the verb (저장하고), and 나서 follows as part of this sequence pattern.

Who is the subject here? Why is it omitted?

Korean commonly drops subjects when clear from context. Here it’s usually “I” or “we.” If you want to show it, add it: 저는 문서를 저장하고 나서 컴퓨터를 꺼요.

What politeness/tense is 꺼요? How do I say it in past/future/command/formal?

꺼요 is polite present (해요체). Variants:

Past: 문서를 저장하고 나서 컴퓨터를 껐어요.
Future/intention: 문서를 저장하고 나서 컴퓨터를 끌 거예요.
Formal statement: 문서를 저장하고 나서 컴퓨터를 끕니다.
Polite request/command: 문서를 저장하고 나서 컴퓨터를 끄세요.
Let’s: 문서를 저장하고 나서 컴퓨터를 끌게요/끕시다.

How does 끄다 become 꺼요?

It’s the ㅡ-vowel rule with 아/어:

끄 + 어요 → 끄어요 → 꺼요 (contraction) Other forms:
Past: 끄 + 었어요 → 끄었어요 → 껐어요
Formal: 끕니다 (no ㅡ deletion with -(스)ㅂ니다)
Imperative polite: 끄세요

Is the 하고 here the same 하고 that means “and” for nouns?

No. Here it’s the verb 저장하- + connective -고 in the pattern -고 나서. The noun connector N하고 N (pen and notebook) is a different 하고.

Verb: 저장하고 나서
Noun: 펜하고 노트

Do I need the object particles 를? Can I drop them?

They’re correct and natural: 문서를, 컴퓨터를. In casual speech you can drop them if context is clear: 문서 저장하고 나서 컴퓨터 꺼요. Keep them when there’s any chance of ambiguity. Reminder: use 를 after a vowel and 을 after a consonant.

Can I rearrange it like 컴퓨터를 문서를 저장하고 나서 꺼요?

That order is unnatural. The -고 나서 clause normally precedes the main clause. For emphasis you could topicalize: 컴퓨터는 문서를 저장하고 나서 꺼요.

Can the subject change between the two actions?

By default -고 나서 implies the same subject. If the subject differs, mark it to avoid confusion or use a form that tolerates subject change better:

동생이 문서를 저장한 후에 제가 컴퓨터를 꺼요.
Or split into two sentences.

Can I use -고 나서 with adjectives (descriptive verbs)?

Generally no. -고 나서 is used with actions. With adjectives, choose other structures (e.g., -게 되다, -아/어서), depending on meaning.

How do I say “right after saving” or “without saving”?

Right after: add an adverb like 바로/곧바로 → 문서를 저장하고 나서 바로 컴퓨터를 꺼요.
Without saving: 문서를 저장하지 않고 컴퓨터를 꺼요.

Can I use 그리고 나서 instead?

Yes, when linking sentences: 문서를 저장했어요. 그리고 나서 컴퓨터를 껐어요. Don’t redundantly use both in one clause; pick either -고 나서 inside one sentence or 그리고 나서 to start the next step.

Is 문서 the same as 파일 here?

Both are fine, but nuance differs:

문서 = a document (e.g., word-processing doc).
파일 = any file. If you literally mean a word-processing document, 문서 is spot-on. Otherwise 파일 is broader.

Any common mistakes to watch out for with this sentence pattern?

Using -고 나서 with adjectives (avoid).
Treating 꺼요 as a polite request; for requests use 끄세요.
Forgetting the participle with 후에/다음에/뒤에: say 저장한 후에, not just 저장 후에 in speech (the short noun form is more written).
Over-scrambling the order: keep the -고 나서 clause before the main action.