Both -아서/어서 and -(으)니까 give a reason ("because, so"). Learn to form each one, and the two rules learners break most: only -(으)니까 can come before a command or suggestion, and no past-tense marker goes before -아서/어서 (no -았/었어서).
Choose between -아서/어서 and -(으)니까 for a reason clause, build the correct form (including nouns and ㄹ/ㅂ irregular stems), and fix sentences that use -아서/어서 before a command/suggestion or with a past-tense marker.
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