第三条件句，即与过去事实相反的条件句(If I had done…, I would have…)

现在完成进行时

过去完成时

间接引语时使用转述动词，如：say , tell , ask , report , advise , promise 等

连接短语，如：even though , in spite of , unless

连接上下文的短语，如：so to continue , in other words , for example