this city is really comfortable to live in.老大今天我在书上看到了这个句子.

这句话是正确的.这是个介词后置到句末的手法,英语当中其实也很常见.
I work FOR this company. It is very nice. 合并后变成：
I work FOR this company which is very nice. 介词后置、更改主语之后变成：
This company is very nice to work FOR.
再来一组和你原句比较像的：
(I, you, or someone) live(s) IN this city. It is very nice. 合并后变成：
(I, you, or someone) live(s) IN this city which is very nice. 介词后置、更改主语之后变成：
This city is very nice to live IN.
仔细比较一下,你应该自己都看得出来啦~