主语从句和宾语从句如何辨别I don't know if you can help me.I didn't know t

主语从句和宾语从句
如何辨别
I don't know if you can help me.
I didn't know that you would come until Friday.
哪句是宾语从句哪句是主语从句?

两句都是宾语从句.这里缺少了know的宾语,一般情况下句子完整的话,都会有know sth.所以这里都是用从句来做know的宾语.
举个主语从句的例子
eg:Whether he will come or not is not sure.这里用whether he will come or not 就充当了主语的成分.
顺便给你一些宾语从句的资料.希望对你有帮助
宾语从句
1.定义：用作宾语的从句叫宾语从句.
2.构成：关联词 简单句.
3.引导宾语从句的关联词有三类：
（1）从属连词that.如：
He told us that he felt ill.他对我们说他感到不舒适.
I know he has returned.我知道他已经回来了.
注： that在引导宾语从句时也并不是任何情况下都可以省略.在以下情况下,that不能省略.
1.Everybody could see what happened and that Tom was frightened.（and连接两个宾语从句,that宾语从句放在and的后面时,that不能省略.）
大家都会看出所发生的事情并知道Tom非常害怕.
2.I know nothing about him except that he is from the south.（that引导的宾语从句作介词宾语时,that不能省略.）
对他我一无所知,只知道他是南方人.
3.That he ever said such a thing I simply don’t believe.（that从句位于句首时,that不可省略.）
我简直不相信他曾说过这样的话.
4.We decided,in view of his special circumstances,that we would admit him for a probationary period.（主句谓语动词与that从句之间有插 入语,that不可省略.）
鉴于他的非凡情况,我们决定应允他一段试用期.
（2）从属连词if/whether.如：
I doubt whether he will succeed.我怀疑他是否会成功.
I don’t know if you can help me.我不知道你能否帮助我.
（3）连接代词who,whom,whose,what,which,whoever,whatever,whichever
连接副词 where,when,how,why.
如：
Who or what he was,Martin never learned.
他是什么人?他是干什么的?马丁根本不知道.
I wonder what he’s writing to me about.我不知道他要给我写信说什么事.
I’ll tell you why I asked you to come.我会告诉你我为什么要你来.
You may do what you will.你可做任何你想做的事.
（1）介词宾语从句
宾语从句也可用作介词的宾语.如：
He was deeply displeased by what had occurred that day.
他对那天发生的事感到很不快.
I walked over to where she sat.我走向她坐的地方. I am curious as to what he will say.我很想知道他要说什么.
Your success will largely depend upon what you do and how you do it.
你是否成功将主要取决于你做什么和怎样做.
有时介词可以省略.如：
I don’t care （for）who marries him.我不管谁跟他结婚.
Be careful （as to）how you do that.你要注重做这件事的方式.
解释：
1.假如宾语从句后还有宾语补足语,则用it作形式宾语,而将宾语从句后置.如：
We thought it strange that Xiao Wang did not come yesterday.
我们认为小王昨天没来是希奇的.
He has made it clear that he will not give in.
他已表明他不会屈服.
2.作介词的宾语：连词that引导的名词性从句很少作介词的宾语,只用在except,but,in后.其他一些介词的宾语从句假如由连词that引导,则需用it先行一步,作形式宾语.如：
He is a good student except that he is careless.
他是一个好学生,只是有点粗心.
You may rely on it that I shall help you.你可以指望我会帮助你的.
介词宾语不可以用which来引导,而要用what来引导.如： Are you sorry for what you’ve done?
你为你所做的一切感到内疚吗?
3.某些形容词或过去分词后常接宾语从句,这类形容词或过去分词有sure,glad,certain,pleased,happy,afraid,surprised,satisfied等,连词that可省略.如：
I am not sure what I ought to do.
我不能确定我该做什么.
I’m afraid you don’t understand what I said.
恐怕你没领会我说的意思.
I’m surprised that I didn’t see all that before.
我好希奇,我以前没看到过.
Mother was very pleased her daughter had passed the exams.
妈妈为她的女儿通过了考试而感到兴奋.
4.连词whether （…or not）或if引导的宾语从句
if和whether引导的宾语从句可以互换使用,但whether常和or not连用,if一般不与or not连用.如：
I wonder whether it is true or not.我不知它是真是假.
用if引导宾语从句假如会引起歧义,应避免使用if而用wheter.试比较：
Please let me know if you want to go.
Please let me know whether you want to go. if从句可理解为宾语从句,意为“请告诉我你是否想去”；此句又可理解为条件状语从句意为“假如你想去的话,请告诉我一声”.
5.宾语从句的否定转移.在think,believe,suppose,expect等动词后的宾语从句,有时谓语尽管是否定意思,却不用否定形式,而将think等动词变为否定形式.如：
I don’t think you are right.我认为你错了.
I don’t believe they have finished their work yet.我相信他们还未完成他们的工作.
I don’t suppose he cares,does he? 我想他不在意,是吗?
6.宾语从句的时态变化规律：
（1）当主句是一般现在时态时,从句可根据需要用任何时态.
（2）当主句是一般过去时态时,从句只能使用过去范围内的任何时态.但客观真理除外.如：
The teacher said that the earth goes round the sun.
老师说地球绕着太阳运行.