只有"先行词+关系代词/关系副词+定语从句"才是一个完整的定语从句，不过，关系代词在定语从句中作宾语或者作表语时可以省略；但若关系代词作介词的宾语、介词又提前的话，关系代词不能省略。
Here is the man (whom/ who/ that) you want to see.这就是你想见的人。
I don't like people who/that get angry easily.我不喜欢容易发脾气的人。
This is the pencil whose point is broken. 这就是那个折了尖的铅笔。
He came back for the book (which) he had forgotten. 他回来取他丢下的书。
He doesn't seem to be the man (that) he was ten years ago.他看起来已经不是十年前的样子了。
The girl to whom I spoke is my cousin. 跟我讲话的姑娘是我表妹。
The tool with which he is working is called a saw.
=The tool (that/ which) he is working is called a saw.他干活用的那个工具叫叫锯。
首先，所有的句子都是完整的句子！
第一句： that 作定语从句的主语，句子是主谓结构。如果加it，就与that重复了(两个主语了)。
你不要忘了：定语从句的引导词是要在句中作成分的。这点与that 引导宾语从句不做成分截然不同。一般that在定语从句中作主语或宾语。如 The boy that likes English is Tom. (that作主语，千万别再加he)，
The bike that you bought is very nice. (作宾语)
第二句：that 作定语从句的宾语。tell 可带双宾语，即tell sb sth，本句中，you (sb) 和 much (sth) 都是宾语。