在某一明确的时间标记之前已经发生的事情,用过去完成时.
象你举的这个例子,应该用过去进行时：he was cooking when I arrived home.
过去完成时：he had cooked before I arrived home
我到家后,它才开始做饭：he began to cook after I arrived home,条件句没有必要用完成时.