Question

Which is the house that he lives这句话中哪个词是先行词？

Answer 1

回答如下：
本句有误，lives应该后跟介词in，或者把关系代词that改成关系副词where，因为live是不及物动词，"住在这座房子里"的英语应该是lived in the house。
本句中的先行词是名词house。先行词就是定语从句所修饰的名词或句子。例如：I know the man (先行词)who is standing under the tree. This is the car (先行词) (which / that ) I bought yesterday. He is injured, which is true. (which 的先行词是前面的整个句子，即He is injured。)

追答

单句这样写是错误的。但是，若有上下文，确实可以这样写。

Answer 2

先行词是the house。
①先行词其实就是定语从句(也叫作“形容词性从句”)修饰的对象。
②从句that he lives修饰the house。