he tells me that he lost his watch last week.
He told me that he had lost his watch last week.
第一句主句是一般现在时,用tells,而从句是last week ,应用一般过去式
第二句主句是一般过去时,用told,从句是过去的过去,用过去完成时.
还是不太懂的话可以再问我O(∩_∩)O~那例如"We weren't ready"这种有be动词的呢？weren't 是过去式，如果用在第一句中做从句的话还是一般过去时，用weren't在第二句的从句中用过去完成时，weren't 改为had not been