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勉強， べんきょう
1. study
2. diligence, working hard
3. experience, lesson (for the future)
4. discount, price reduction
学习
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なくてはいけない， 無くてはいけない
1. cannot do without something, indispensable, absolutely necessary
2. have to do
tbd
は
1. indicates sentence topic
2. indicates contrast with another option (stated or unstated)
3. adds emphasis
指示句子的主题；表示与另一选项（无论是否明确提及）形成对比；表示强调
