Without your help,they could not finish the work so soon.
简单句通常为只有一个主语,谓语,宾语等.如：I like reading.
而复杂句则为两个以上主语,谓语等.如例句中：they could not finish the work so soon 为一个句子,其中they为主语,而if you did not give them so much help 为另一个句子,其中you为主语.