非限制性定语从句
非限制性定语从句既可修饰先行词，也可修饰整个主句，起补充说明作用，与主句之间有逗号隔开，若省去，原句意思不受影响。
非限制性定语从句不能用that引导。关系词不可省略。
例如：Our guide，who was a French Canadian，was an excellent cook.
Mr. Smith，from whom I have learned a lot，is a famous scientist.
The boy, whose father is an engineer, studies very hard.
He said that he had never seen her before，which was not true.
They went to London，where they lived for six months.
注意：以上例句中，均未用that引导非限定性定语从句。
When deeply absorbed in work，which he often was，he would forget all about eating and sleeping.
此句中，whichhe often was 是非限定性定语从句，修饰When deeply absorbed in work整个句子。
“介词 + which / whom / whose”引导的定语从句
“介词 + which / whom / whose”可引导限制性定语从句，也可引导非限制性定语从句，该结构中介词的选择取决于从句谓语动词的固定搭配，或先行词的习惯搭配。
例如：This is the computer on which he spent all his savings
It is written by a person with whom we are all familiar.
由as引导的定语从句
as引导的定语从句主要用于“such...as”及“the same...as”的结构中，代替先行词是人或物的名词。as引导非限制性定语从句时，代替整个主句，从句可位于主句之前、之后或中间。通常由逗号将其与主句隔开。as有“正如……， 就像……”之意。
例如：These are not such problems as can be easily solved. (as代替先行词problems)
As is mentioned above, no single company or group can control what happens on the Internet. (as代替主语)
As is known to the United States, Mark Twain is a great American writer.（as在从句中作主语）
He forgot to bring his pen with him, as was often the case.（as在从句中作主语）
He is absorbed in work, as he often was.（as在从句中作表语）
Boy as he was，he was chosen king.
他虽是孩子，却被选为国王。（as在从句中作表语）
As we all know, the earth is round.
众所周知，地球是圆的。（as在从句中作宾语）
The two brothers were satisfied with this decision，as was agreed beforehand.
兄弟俩对此决定都满意，这项决定事前都已得到他们的同意。(as在从句中作主语)
Taiwan is，as you know，an inseparable part of China.
正如你所知，台湾是中国不可分割的一部分。(as在从句中作宾语)
