由as if或as though引导的状语从句表示比较或方式时.从句谓语形式为动词的过去式（be用were）或“had十过去分词 那主句的时态怎么确定.例子!

主句的时态用现在时从句就用过去时.主句用过去时从句用过去完成时.如
The old man walks as if he were a young man.
He talks as if he knew everything.
He acted as if nothing had happened.