-ㄴ/는/ㄹ 줄 알다

This versatile pattern has two distinct meanings depending on the context and conjugation. Understanding both is key.

1. Ability: 'To know how to'
This form is used with action verbs to express knowledge of a skill or method. It always uses the future tense modifier -ㄹ/을.

• Verb stem ending in a vowel or the consonant ㄹ + -ㄹ 줄 알다 (e.g., 운전하다 → 운전할 줄 알다; 만들다 → 만들 줄 알다)
• Verb stem ending in a consonant (except ㄹ) + -을 줄 알다 (e.g., 먹다 → 먹을 줄 알다)

This is similar to -ㄹ 수 있다 ('can'), but -ㄹ 줄 알다 specifically emphasizes the knowledge or skill required, whereas -ㄹ 수 있다 can also mean possibility or permission.

2. Assumption: 'To think that...' (often mistaken)
With this form, 알다 means 'think' or 'assume', not 'know a fact'. The conjugation depends on the verb/adjective type and tense.

• Action Verbs (Present): Verb stem + -는 줄 알다
  (e.g., 그 사람은 자기가 다 아는 줄 알아요. - He thinks he knows everything.)
• Descriptive Verbs (Present): Adjective stem + -ㄴ/은 줄 알다
  (e.g., 다들 제가 어린 줄 알아요. - Everyone thinks I'm young.)
• Past Tense (Action Verbs): Verb stem + -(으)ㄴ 줄 알다
  (e.g., 그가 이미 떠난 줄 알았어요. - I thought he had already left.)
• Future Tense (Verbs/Adjectives): Stem + -ㄹ/을 줄 알다
  (e.g., 그가 내일 올 줄 알았어요. - I thought he would come tomorrow.)
• Nouns (Present): Noun + -인 줄 알다
  (e.g., 오늘이 휴일인 줄 알았어요. - I thought today was a holiday.)

Note that the future tense form for 'assumption' is identical to the form for 'ability'. The meaning is determined by the context and the verb used. For example, with verbs like 오다 (to come) or 춥다 (to be cold), it expresses an assumption about the future, not a skill.

Important Nuance: Mistaken Assumption
In the present tense, 알다 means 'think' or 'assume' (내가 바보인 줄 알아? means 'Do you think I'm stupid?'). When this pattern is used in the past tense (알았다), it very often implies that the speaker's assumption was incorrect. It translates to 'I thought that...', suggesting a mistaken assumption that has now been corrected.
e.g., 시험이 오늘인 줄 알았어요. - 'I thought the test was today.' (but it's not).

알았다 can also mean 'I knew' when the guess turned out to be right: 그럴 줄 알았어요. - 'I knew it.' 몰랐다 means 'I didn't realize': 네가 이렇게 바쁜 줄 몰랐어. - 'I didn't know you were this busy.'

To say that you know a fact, use -ㄴ/는 것을 알다 (오늘이 일요일인 것을 알아요). To say that you know whether, who, or what, use -ㄴ/는지 알다 (오늘이 무슨 요일인지 알아요).