Learn Hardcore Korean: geunyeoneun jajeongeoreul tal jul ara.

It means “to know how to V.” 줄 is a bound noun meaning “way/skill/method,” and 알다 is “to know,” so 탈 줄 알아 = “(she) knows how to ride.” The common negative is V-(으)ㄹ 줄 모르다 (e.g., 탈 줄 몰라 = “doesn’t know how to ride”).

Why does the sentence end with 알아? Can I make it polite?

알아 is informal/casual (banmal). Polite forms:

Standard polite: 알아요 → 그녀는 자전거를 탈 줄 알아요.
Formal polite: 압니다 → 그녀는 자전거를 탈 줄 압니다.

What’s the role of 는 and 를 here? Could I use 가 instead of 는?

는 marks the topic: 그녀는 sets “she” as what we’re talking about.
를 marks the object: 자전거를 is what she rides.
그녀가 자전거를 탈 줄 알아 is possible; 가 emphasizes that she (not someone else) is the one who knows, or presents her as new info. With neutral, general statements, 는 is more typical.

Why is it 탈 줄 and not 타 줄 or 타는 줄?

탈 is the adnominal “future/intentive” form of 타다 (ride) used before 줄: 타다 → 탈 줄.
타 줄 would be interpreted as “ride (for someone)” using the -아/어 주다 (“do for”) pattern, which is different.
타는 줄 means “(I) think/assume that (someone) rides,” not “know how.” So for ability, use V-(으)ㄹ 줄 알다.

Could I say 자전거를 탈 수 있어요 instead? What’s the nuance difference?

V-(으)ㄹ 줄 알다: knows how (has learned the skill).
V-(으)ㄹ 수 있다: can/is able to (possibility/ability/permission). Example: With a broken arm, she might still “know how” (탈 줄 알아요) but currently “can’t” (탈 수 없어요).

How do I make the negative, and what’s the difference among 몰라요 / 못 타요 / 탈 수 없어요?

탈 줄 몰라요: doesn’t know how (hasn’t learned the skill).
못 타요: can’t ride (due to inability, condition, or circumstance).
탈 수 없어요: cannot ride (impossible/not allowed/no permission).

Is 그녀 natural in everyday Korean?

Not usually. 그녀 (she) appears in writing, translations, lyrics, etc. In conversation people prefer a name/title (민지 씨), or pronouns like 그 사람 (that person), 그 여자 (that woman), or colloquial 걔 (that kid/person). Often the subject is dropped: 자전거 탈 줄 알아요.

Do I need the object particle 를 after 자전거? Can I drop it?

를 is correct, but in casual speech objects are often unmarked, so 자전거 타요 is very common. With or without 를 both are natural here. Keep 를 in careful/formal writing.

Why not 자전거에 타다? What’s the difference between -를 타다 and -에 타다?

N를 타다 = ride N (as a means of transport): 자전거를 타다.
N에 타다 = get on/be on N (state of being on): 자전거에 타다 “to get on/be on a bike,” not the act of riding it. For “ride,” use -를 타다 (often spoken without 를).

Any pronunciation tips?

알아 is pronounced like [아라] because ㄹ links to the following vowel.
줄 알아 links across the space and sounds like [주라라].
Full phrase (one natural reading): [그녀는 자전거를 탈 줄 아라] ~ [그녀는 자전거를 주라라] depending on liaison; don’t overthink it—aim for smooth linking.
A rough romanization: geunyeoneun jajeongeoreul tal jul ara.

How should I space it? Is it 탈줄 or 탈 줄?

Write 탈 줄 with a space. 줄 is a dependent noun, so it’s spaced off the verb form before it. Also: 알아 is one word, and particles attach with no extra space: 자전거를.

How do honorifics work if the subject is someone I should respect?

Common options:

자전거 탈 줄 아세요? (honorific on 알다)
자전거 타실 줄 아세요? (honorific on the embedded verb) — widely used in speech
Formal: 자전거 탈 줄 아십니까? If you’re describing a respected third person: 그분은 자전거를 탈 줄 아세요.