句型一：过去完成时+when+一般过去时 如：
I had finished my homework when Mum came back.妈妈回来的时候,我已经完成我的作业了.
句型二：过去完成时+before+一般过去时 如：
I had finished my homework before Mum came back.妈妈回来之前,我已经完成我的作业了.
句型三：after+过去完成时+一般过去时 如：
After I had finished my homework,Mum came back.我已经完成我的作业之后,妈妈才回来.