Having finished the homework,he went home.为什么用现在完成时不是过去完成时

Having finished the homework,he went home.
Having finished the homework,这个不是现在完成时,而是现在分词的完成式,充当状语.表示这个动作发生在主要动作（went home）之前.
做完作业,他回家了.
Having finished the homework
=After he had finished the homework