只用that不用which的情况是什么？
只能用that，不能用which作先行词的情况：
（1）先行词是不定代词或者先行词被不定代词所修饰时，关系代词通常只用that，不用which。
（2）先行词被序数词、形容词最高级、the only，the very，the last等修饰时，关系代词通常只用that，不用which。
（3）先行词既有人又有物时，关系代词只用that，不用which。
（4）关系代词在限定性定语从句中作表语并带有类比含义时，通常只用that，不用which。
That和which在定语从句中的用法区别
1、可以用介词加which，不能用介词加that。
Your metabolic rate is the speed at which your body transforms food into energy.
新陈代谢的速度是你的身体将食物转化为能量的速度。
Behavior is a mirror in which everyone shows his image.
行为是一面镜子，每个人都把自己的形象显现于其中。
2、非限制性定语从句里，用which不用that。
Unprofitable eloquence is like the cypress, which is great and tall, but bears no fruit.
夸夸其谈不济事，柏树参天不结果。
It's raining, which probably means that the football match will be cancelled.
下雨了，这可能意味着足球赛将被取消。
3、如果先行词在从句中做宾语，that/who/whom/which都可以省略。
There are two points (which/that) I wanted to make.
我想说两点。
The cell is the unit (that/which) all living organisms are composed of.
细胞是构成所有生物的单位。
