一、定语从句引导定语从句的关系代词有that, which, who （宾格whom，所有格whose）和关系副词where when why 等，关系代词或关系副词在定语从句中充当一个成分，关系代词that，which,who,whom等在从句中分别作主语或宾语，whose 在从句中作定语，而关系副词when,where,why等在从句中作状语。
如：①I will never forget the days when/in which we worked together.②I will never forget the days which/that we spent together.解析：在句①中，表示时间的名词the days在从句中充当的是状语，所以用关系副词when来代指，引导定语从句修饰先行词the days；而在句②中，表示时间的名词the days在从句中充当的是动词spent的宾语，所以用关系代词that 或which来代指。
同样，表示地点或原因的名词如果在从句中作状语，则用关系副词where 或why来代指；如果在从句中作动词的宾语，则用which或that来代替。
如：①This is the factory where/in which I worked.（作状语）②This is the factory that/which I visited years ago.（作宾语）注：当先行词为time,reason, place时，引导词可以省略。
如：① This was the first (when/what) I had serious trouble with my boss.② That is the reason (why) I did it.③ This is the place (where) we met yesterday.另外，定语从句中谓语动词的数应与先行词的数相一致。
如：① Mr. Jackson is the only foreigner that is present at the party.0② He is one of the students who were praised by the teacher.解析：在句①中，先行词foreigner被only修饰，强调只有一个，所以从句中谓语动词用单数形式，而在句②中，who引导的定语从句修饰先行词the students，为复数，所以从句谓语动词应为复数。
感悟：学生之所以在这一点上经常出错，主要是对定语从句的理解不够深刻，分辨不出主句和从句，对句子成分的把握也不到位，搞不清关系代词或关系副词在从句中充当何种句子成分。
所以要做好这一类题，要首先弄清关系代词和关系副词的用法，而且应善于分辨和分析句子结构及其相应成分，这样才能对症下药，快速准确地找到瓿之所在。
二名词性从句中的易错点（一）that引导的定语从句与同位语从句定语从句与前面的的名词是修饰与被修饰的关系，“……的名词”，而同位语从句是用来说明前面名词的内容。
与that从句同位的名词必须是一些表示事实或概念的抽象名词，如fact,news,belief,truth,reply等。
That在定语从句中必须作成分，可用which或who/whom代替，而that在同位语从句中不充当任何句子成分，只起连接作用。
如：①Along with the letter was his promise that he would visit me this comi ng Christmas. ②Do you still remember the chicken farm that we visited three months ago.解析：在句①中，that引导同位语从句解释说明promise的内容，that不充当任何句子成分，只起连接作用，而在句②中，that在其引导的定语从句中作动词visited的宾语，对先行词the chicken farm起修饰作用。
感悟：学生之所以在这一点上容易出错，主要还是因为对定语从句的理解不够深刻，分辨不出that在从句中是否充当了句子成分，因此，还需在句子分析上下功夫，并且搞清何谓同位语从句。
（二）名词性从句中，关于it作形式主语和形式宾语的易错点1. 名词性从句作主语或宾语时，为保持句子平衡，常用it作形式主语或形式宾语。
如：It’s a pity that he don’t come to give a speech.（形式主语）We think it possible that you can finish the job today.（形式宾语）2）谓语动词appreciate, dislike, hate, like, love, make等接由if或when 引导的宾语从句时往往在从句前加上形式宾语it. 例如：I would appreciate it if you could come to my birthday party.3）动词hare, take, hide, punish, put等,后接由that 引导的病因从句式，往往在从句前加形式宾语it.例如：① I take it that you will be leaving Shanghai soon.② we punished it that we had finished the project ahead of time.4）短语动词answer for, count on depend on, insist on, see to等后接有th at引导的宾语从句时，必须冠以形式宾语it.例如：① I’m counting on it that you will come.② She’ll see to it that he goes ahead.注：作形式主语和形式宾语时只能用it.三、代词it、one、that的用法与区别.One泛指，指代前面提到的那类人或物，用于指代可数名词，相当于a+名词单数，其复数形式ones前一般要有定语，否则就要用some.That（those复数）即可指代不可数名词，表特指，相当于the+名词，后面经常与介词修饰语连用。
It指代上文出现的名词，表示同类同物，另外还可表示时间、距离、天气、还可指代人称代词，表示性别身份不明，可作形式主语，形式宾语，引导强调句式以及一些固定搭配，如：get it, catch it, make it.例如：① I have lost my watch. I think I must buy one.我丢了只表，我想我必须再买一只。
（代指上文指到的同类事物，但不指同一个）② Where is my pen? Have you seen it? 我的钢笔不知弄到哪去了，你见了吗？（代指上文提到的同一事物）③ The land of China is larger than that of America.④ Tome has a red pen and a blue one （或two blue ones）⑤ He has no child, and he wants to adopt one （或some）四、虚拟语气I 虚拟语气在if 引导的条件句中的易错点。
① if my lawyer had been here last Saturday, he would have prevented me from going.② if you had studied hard at school, you would be a college student no w.句子①②都是虚拟语气在条件句中的使用，从句中都是假设过去的情况，所以用过去完成时，但在主句中，句①依然指代过去的情况，谓语动词是would h ave done，而②中含有一个表示现在的时间状语now, 这样就必须将其理解为假设现在的情况，所以谓语动词必须为would/should/might+动词原形，学生在这一点上经常会忽略now的存在，从而按句①的形式填写答案。
Ⅱ虚拟语气在名词性从句中的用法1.在it is important (strange, natural, necessary……)+that 句子或者It is de cided (ordered, suggested, demanded, advised……)+that句子中，主语从句中的谓语动词常用（should）+动词原形结构，表示某事“重要”、“必要”、“被决定”等例如：① it is important that everyone (should ) obey the rules all day and all night.② it is decided that the meeting (should) be held tomorrow afternoon.2. suggest, insist 后面指宾语从句时需注意的地方1）①suggest 当“建议”“提出”讲时，其后宾语从句的谓语动词常用“sh ould+动词原形”例如：He suggested that the work (should) be started at once 他建议立即动工。
类似的动词还有insist坚持，demand要求，desire要求、请求，request请求，require要求、需要，order命令，propose建议，command命令，ask要求，advis e建议，prefer宁愿等。
这些动词变被动语态（如：It is suggested + that主语从句）形式后，主语从句谓语动词仍用“（should）+动词原形”这些动词变名词（如suggestion）后，其后的表语从句和同位语从句中的谓语部分仍用“(should)+动词原形”②suggest 当“提出（某看法），暗示，启发”讲时，其后宾语从句的动词不用虚拟语气。
例如：（1）The police suggested that the thief might be one of the fami ly member警察局提出窃贼可能是一名家庭成员。
（2）Her yawns suggests that she is sleepy. 她打哈欠表明她困了。
（3）Although he didn’t suggest that we __ the decision to swim across the river, but his look suggested that our decision __wrong.虽然他没提出我们应该终止游过这条河的决定，但他的表情表明我们的决定是错误的。