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1. V+ㄹ/을 N : A Noun that will do Verb in the future
e.g. 잘 사람: Person who will sleep
and as you might know, whether to use between ㄹ and 을 depends on the existence of a final sound of V(stem) you're using.
자다(to sleep)->stem of this word is 자, and it has no final sound, so you will add ㄹ below 자, which becomes 잘. If is was 먹다(to eat), then it would be 먹(stem, but with a final sound)+을, and become 먹을.
2. V+는 N : A Noun that is currently doing Verb
e.g. 자는 사람 : Sleeping person
Adjectives go by this rule too.
