Since: -(으)니까

-(으)니까 gives a reason, like -아/어서, but it is the one to use when the second clause is an order, request or suggestion (-세요, -아/어 주세요, -(으)ㅂ시다, -자, -(으)ㄹ까요).

• Stem ends in a vowel: -니까 (오다 → 오니까)
• Stem ends in ㄹ: the ㄹ drops, then -니까 (멀다 → 머니까, 알다 → 아니까)
• Other consonants: -으니까 (없다 → 없으니까)
• ㅂ-irregulars: 춥다 → 추우니까
• Nouns: (이)니까 (주말 → 주말이니까)

Unlike -아서, it can take the past: 버스가 떠났으니까 다음 버스를 기다리세요 (The bus has left, so wait for the next one).

비가 와서 우산을 가져가세요 is wrong; say 비가 오니까 우산을 가져가세요 (It's raining, so take an umbrella).