Hi, This is because 먹다 and 사다 are verbs, while 작다 is actually an adjective. (Your guess is right. It has to do with action words / non-action words). For verbs, you always put -는지 whether it ends in a consonant or a vowel. 하다 - 하는지 먹다 - 먹는지 When it ends in ㄹ, however, ㄹ is dropped. 살다 - 사는지 줄다 - 주는지 For adjectives, you put -ㄴ지 (if it ends in a vowel), or -은지 (if it ends in a consonant). 작다 - 작은지 예쁘다 - 예쁜지 if it ends in ㅂ, then you drop the ㅂ and put -운지 아름답다 - 아름다운지 차갑다 - 차가운지 There's really no way to distinguish verbs from adjectives by just looking at them. But if you know the meaning, it's usually pretty clear.