定语从句是修饰名词或代词的从句，定语从句所修饰的词叫先行词。关系代词或关系副词指代先行词，在从句中充当一定的成分。所以，不存在定语从句不缺成分的情况。
你所说的是同位语从句，若从句中不缺成分，用that引导，且不能省略（除在宾语从句中可省略）
在名词从句中,除了that /whether / if 只起连接作用,不能充当任何成分外,其他连接词都在从句中充当一定的成分解题的关键就是“缺什么补什么”,而正确解题的前提即是“如何判断缺成分,缺了什么成分”首先判断从句中缺少的是主语、宾语还是状语,判断的依据是从句中谓语动词前有无主语,或者谓语动词后有无宾语,如果是这二者之一,那么你会判断应该使用连接代词what / whatever ; who / whoever ; which 通常情况下,选择what 的概率很高如果从句中不缺少主语或者宾语,那么就一定缺少状语,状语的判断主要依据句意,常考的是when / where 如果依然看不懂,那么你可以先试试复习一下五种简单句的句型,也许会有莫大的帮助
一、状语从句主谓成分的省略，须遵循下面原则：
（一）当从句的主语与主句的主语一致时，从句可以省略主语，从句的谓语动词采用其V-ing结构。例如：
Although working very hard, he failed in the final exam (Although he worked very hard,…)
After taking the medicine, she felt much better (After she took the medicine,…)
If going there by air, we’ll have to pay twice the fare (If we go there by air,…)
（二）当从句的主语和主句的主语一致时，若从句的谓语动词是系动词be或包含be的某种形式，此时从句的主语和be动词通常可以一同省略。常见于以下几种形式:
1、连词+形容词
Whenever (she is) free, she often goes shopping她有空就去逛商店。
Work hard when (you are) young, or you'll regret少壮不努力,老大徒伤悲。
She hurried out of the room, as if (she was) angry
2、连词+名词
While (he was) a young boy, he was always ready to help others他在孩提时代就乐于助人。
He could write poems when (he was) yet a child
3、连词+现在分词
While (she was) walking along the river bank, she was singing a pop song她沿着河堤边走边唱着流行歌曲。
The boy is running impatiently here and there as if (he is) searching for something lost on the sports ground 这男孩很不耐心地在操场上到处跑着仿佛在找什么东西。
4连词+过去分词
If (it is) well read, the book will give you much to think
Unless (they were) asked to answer questions, the students were not supposed to talk in Prof Li’s class
5连词+不定式
He stood up as if (he were) to say something当时他站起来好像要说什么。
He wouldn't solve the problem even if (he were) to take charge即使他来负责,他也解决不了这个问题。
6、连词+介词短语
She looked anxious as though (she was) in trouble她看上去很焦急,好像遇到了麻烦。
He looked everywhere as if (he was)in search of something 他到处看似乎在找什么东西。
He came across the picture while (he was) on a visit to New York
（三）若从句的主语和主句的主语不一致，但和主句的宾语一致时，从句中的主语和be动词亦常可省略。例如：
Father advised me not to say anything until (I was) asked
Granny told the children not to talk while (they were) eating
（四）当从句的主语是it, 且谓语动词是be或包含be的某种形式时, it和be常可同时省略。例如:
If (it is) possible, he will help you out of the difficulty如果可能的话,他会帮你摆脱困境。
You must attend the meeting unless (it is) inconvenient to you除非情况对你来说不方便,否则你必须出席这次会议。
定语从句缺主语代替人who/that 代替物which/that
定语从句缺宾语代替人whom/that代替物which/that
定语从句缺补语代替人that 代替物that
定语从句不缺成分,先行词表示地点用where
原因用why
时间用when
方式用the way that
宾语从句：引导陈述句用that
引导一般疑问句用if
引导选择疑问句用whetheror
引导特殊疑问句做主语用who what
做宾语用whom what
做补语用who what
做定语用whose what which
做状语用where when why how
引导感叹句用原感叹词或变成that从句
引导祈使句用to do 或变成that从句
宾语从句定语从句引导词的区别
what who when why where whose引导宾语从句时有疑问意义,引导定语从句时无疑问意义
定语从句中，也有句子本身的结构成分，“主谓（宾）、定状补”。首先虽作为一个从句但本身也是一个句子，所以主语、谓语必须具备。是否缺宾语则根据从句中谓语动词是否为及物动词。
判断从句中结构成分的缺失，首先得根据句子的基本结构，即“主谓(宾)”其次则根据句意理解而判断是否缺是该有的修饰成分，如状语就作为句子补充说明成分中的重要成分。
例：
主语：I like the girl who is very kind(who)
宾语：I have finished reading the book my father bought several weeks ago（宾语从句中，宾语 that/which 可省略。）
状语：The factory has been closed three months ago where I worked before(where)
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