hat用法真有趣， 兩個地方它不去;

逗號后邊它不去， 介詞之后不考慮;

That做為關系代詞，不可用在非限制性定語從句中，即不可用在逗號后邊;定語從句中介詞之后也不可使用。在逗號或介詞后，指物時用which,指人時用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：在介詞of之后指物，選擇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：選擇 D whom。介詞后指人。

例句 3)The letter is from my sister, ______is working in Beijing.

A. which B. that C. whom D. who

解析：選擇 D who。逗號后指人，在定語從句中做主語。