since， because， when 都是连词，连接从句，because意思是因为，引导原因状语从句。 since和 when都引导时间状语从句，前者意思是自从，后者意思是当。。。的时候。例句： He was late because he got up late. We have been good friends since we met the first time. Don't get off the bus when it is still going.