现在完成时可以转化成一般过去时，还可以转化成“It is/has been +时间段+since从句(用一般过去时)”等句型。
e.g. Jim has been back for a month. 吉姆已经回来一个月了。 = Jim came back a month ago. 吉姆一个月前就回来了
= It is/has been a month since Jim came back. 自吉姆回来已经有一个月了。 = A month has passed since Jim came back. I have kept the book for two days. = I borrowed the book two days ago.
= It is / has been two days since I borrowed the book. = Two days has passed since I borrowed the book.
