the question is too diffictllt for me to___.答案为answer ,为何不用answer it?同样,i need a room to live in.此处却要加个in,为何不表述为i need a room to live.

the question is too diffictllt for me to___.答案为answer ,为何不用answer it?同样,i need a room to live in.此处却要加个in,为何不表述为i need a room to live.
the question is too diffictllt for me to___.答案为answer ,为何不用answer it?
同样,i need a room to live in.此处却要加个in,为何不表述为i need a room to live.

the question is too diffictllt for me to___.答案为answer ,为何不用answer it?同样,i need a room to live in.此处却要加个in,为何不表述为i need a room to live.
很简单
to live in a room  不是 to live a room
to answer a question, 不是 to answer a question it
你举的例子中,question 和 room 叫做 antecedant, 也就是说是后面动词和介词的宾语.
再举两个例子：
i need someone to love = i need to love someone
i need someone to love me = i need to be loved by someone

动宾搭配answer the question， the question放在前面做了主语，同时又是 answer的宾语，这种情况下，需省略。
live in a room中，live是不及物动词，不能说成 live a room ，必须加介词in。

加了it就和前面的the question 重复了，这是英语中的适当省略避免重复的用法。
而下面那个live in 在句子中是词组，in是介词，不能省略