词汇用法和语法

(1) 引导非限制性定语从句，可以是主句中的名词、形容词、短语、其他从句或整个句子。

These apple trees, which I planted three years ago, have not borne any fruit. (宾语)
She is an artist, which I am not. (表语)
Water，which is a clear liquid, has many uses. (主语)
The two policemen were completely trusted, which in fact they were.
She was very patient towards the children, which her husband seldom was. (指代形容词)
She is always careless, which we should not be.
He said that he had never seen her before，which was not true. (指代主句中的某个从句)
In front of so many people he was nervous, which was understandable.
the second term, in which p denotes the momentum of an electron, is that of magnetic dipole (M1)
Presently, the word luminescence is defined as a phenomenon in which the electronic state of a substance is excited by some kind of external energy and the excitation energy is given off as light.
The VUV levels of many of the lanthanides have been recently measured, thereby providing the starting point from which new phosphors may be designed.
it is the 3P and 1I6 metastable states from which quantum splitting must occur.
Emission occurs when thermal energy raises the electron to a state from which it can de-excite.
This morning some port wine came, for which I have to thank you.
She may be late, in which case we ought to wait for her.
The documents for which they were searching have been recovered.

(x) which和that用法的区别

引导非限制性定语从句，通常要用which。
当先行词是下列不定代词或被它们修饰时much, little, none, all, few, every(thing), any(thing), no(thing) 等时，通常用that。
当先行词有the very, the only, the same 等修饰时，通常用that。
当先行词有形容词最高级或序数词(包括last, next等)等修饰时，通常用that。
当关系代词在定语从句中用作表语时，通常用that。
当关系代词在定语从句中用作表语时，通常用that：
China is not the country (that) it was.