that用法真有趣，两个地方它不去；
逗号后边它不去，介词之后不考虑；
That做为关系代词，不可用在非限制性定语从句中，即不可用在逗号后边；定语从句中介词之后也不可使用。在逗号或介词后，指物时用which,指人时用who或whom。
例句（1）They arrived at a farmhouse, in front of ______sat a small boy.
A. whom B. who C. which D. that
解析：在介词of之后指物，选择C which。
例句（2）The engineer with _____my father works is about 50 years old.
A. whose B. who C. which D. whom
解析：选择 D whom。介词后指人。
例句（3）The letter is from my sister, ______is working in Beijing.
A. which B. that C. whom D. who
解析：选择 D who。逗号后指人，在定语从句中做主语。
