Learn Hardcore Korean: unjeonhaneura jeonhwareul mos badasseo. - I couldn’t answer the phone because I was driving.

-느라 links two clauses and means “because (I was) doing X, [I couldn’t/didn’t] do Y.” It implies the speaker was occupied with the first action and, as a result, the second clause is a negative, failed, or undesirable outcome. It also suggests the two actions overlapped in time.

Is -느라 the same as -느라고?

Yes. -느라 and -느라고 are interchangeable here. -느라고 is the fuller form that’s very common in speech; -느라 is a slightly shorter variant. Meaning and usage are the same in sentences like this:

운전하느라(고) 전화를 못 받았어.
운전하느라고 전화를 못 받았어.

Why not use -아서/어서, -(으)니까, or -(으)기 때문에?

All of these can express cause, but the nuance differs.

운전해서/운전하니까/운전하기 때문에 전화를 못 받았어. Neutral “because.”
운전하느라 전화를 못 받았어. Emphasizes being tied up doing something and strongly expects a negative/failed result. Sounds apologetic/explanatory.

Also, -아서/어서 and -(으)니까 work with adjectives too; -느라 is mainly for action verbs.

Can I say 운전해서 전화를 못 받았어 instead? What’s the nuance difference?

Yes. 운전해서… = neutral “because I was driving.”
운전하느라… adds the nuance “I was busy/occupied driving (so I couldn’t).” It’s a more natural choice for excuses/apologies.

How do I form -느라? Any irregulars to watch out for?

Attach -느라 to the verb stem of an action verb:

하다 → 하느라 (운전하다 → 운전하느라)
먹다 → 먹느라
씻다 → 씻느라
기다리다 → 기다리느라

With common irregulars, there’s typically no change because the suffix starts with a consonant:

듣다 → 듣느라 (not 들으느라)
걷다 → 걷느라
돕다 → 돕느라

For 하다-compounds, both 운전하느라 and 운전을 하느라 are fine.

Can I use -느라 with adjectives?

Generally no; -느라 is for action verbs. There are a few idiomatic exceptions when the adjective implies being tied up, especially 바쁘다:

바쁘느라 전화를 못 받았어. (I was busy so I couldn’t answer.) For most adjectives, use -아서/어서 or -(으)니까:
피곤해서/아파서 못 갔어. (Avoid 피곤하느라/아프느라 in standard usage.)

Can the result clause be a command, suggestion, or future plan?

No. -느라 explains a negative/undesirable result; it doesn’t combine with imperatives or suggestions.

Natural: 회의하느라 늦었어요. (I was in a meeting, so I was late.)
Unnatural: 회의하느라 일찍 오세요. (Don’t use -느라 with commands.)

Do the two clauses need to have the same subject?

Yes. The subject of the -느라 clause and the result clause should be the same.

Natural: 내가 운전하느라 전화를 못 받았어.
Unnatural: 친구가 운전하느라 나는 전화를 못 받았어. Use a different connector when subjects differ:
친구가 운전해서 나는 전화를 못 받았어. Or rephrase to keep the same subject:
내가 친구를 기다리느라 전화를 못 받았어.

What’s the difference between 못 and 안 in 못 받았어?

못 받았어 = “couldn’t answer” (inability or external circumstances, e.g., driving).
안 받았어 = “didn’t answer” (volitional choice). For an apology/excuse, 못 is appropriate.

Why is it 전화를 받다? Is that the set expression for “answer a call”?

Yes. Common phone collocations:

전화를 받다 = answer/take a call
전화가 오다 = a call comes in
전화를 하다/걸다 = make a call
통화하다 = be on a call

Do I need the object marker -를 after 전화?

In careful speech/writing, yes: 전화를 못 받았어.
In casual speech, dropping it is common: 전화 못 받았어. Both are fine in conversation.

How would I say this politely or formally?

Polite: 운전하느라 전화를 못 받았어요.
Formal: 운전하느라 전화를 못 받았습니다.
More apologetic: 죄송합니다. 운전 중이라(서) 전화를 받지 못했습니다.

Can I use the -느라 clause by itself as an excuse?

Yes. Elliptical use is natural in conversation:

운전하느라(고요). / 운전하느라고요. (…I was driving.)
Tone supplies the apology.

How is -느라 different from -다가?

-느라 marks reason with an “occupied” nuance: 운전하느라 전화를 못 받았어. (Because I was busy driving…)
-다가 marks that one action was in progress when another happened; it’s more “while/and then” than “because”:
- 운전하다가 전화를 못 받았어. = While I was driving, I didn’t pick up. (Timing emphasized; cause only implied.)

Any other natural ways to say the same idea?

운전 중이라(서) 전화를 못 받았어요. (neutral)
운전하고 있어서 전화를 못 받았어요. (progressive)
그때 운전 중이었어요. 그래서 전화를 못 받았어요. (two sentences, very clear)

What tense/aspect nuance does -느라 carry?

The -느라 clause is tenseless and backgrounded as the reason; it implies the action was ongoing at the time of the second clause. The tense typically appears in the result clause (here, past: 못 받았어).