if you are late,you will not find a seat是一个主语从句 所以是主将从现?

题目

if you are late,you will not find a seat是一个主语从句 所以是主将从现?
要是虚拟语气If I were you,I would have.这不是主语从句么?
我想知道怎么判断主将从现句。有人说是主语从句才能引导主将从现！

答案

主语从句(Subject Clause)
定义:如果一个句子在复合句中充当一个主语,那么这个句子就是主语从句.
第一部分：常规主语从句,即句子在复合句中充当一个主语
（1）That he finished writing the composition in such a short time surprised us all.
（2）Whether we will go for an outing tomorrow remains unknown.
（3）Who will be our monitor has't been decided yet.
（4） Whom we must study for is a question of great importance.
（5）What caused the accident remains unknown.
（6）Whatever you did is right.
（7）Whose watch was lost is unknown.
（8）What we need is time.
（9）What we need are good doctors.
小结:
（1)引导主语从句连词有that,whether,who,what,whatever等
（2)连词位于句首不能省略
（3)主语从句大多数情况下视为单三,但也有例外,如例9
第二部分：为了防止句子头重脚轻,通常把形式主语it放在主语位置,真正主语搁置于句末
（1）It is certain that he will win the match.
（2）It is true that he has made a very important discovery in chemistry.
（3）It is very likely that they will hold a meeting.
（4）It is strange that he should do that.
（5）It is important that we all should attend the meeting.
（6）It is strange that the man should have stuck to his silly ideas.
（7）It is a pity that we won't be able to go to the south to spend our summer vacation.
（8）It is still a mystery what caused the accident.
（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)
（10）It is known to all that the gun powder was first invented by the Chinese.
（11）It is suggested that the work should be done with great care.
（12）It seems that he has seen the film.(=He seems to have seen the film)
（13）It happened that the two cheats were there.(=The two cheats happened to be there)
小结:
（1)以that 引出的主语从句,常以形式主语it引导.It is +形容词/名词/某些动词ed + that 从句.
（2) 在有些that从句中要用虚拟语气 (should+do/should+have done)例句4,5,6,11.