这里的 which 是关系代词。“介词 +  which + 动词不定式” 相当于形容词短语，由定语从句简化而来。例如：

        The poor man has no house in which he can live. =  The poor man has no house in which to live. （ in 必须放在which前面，不能放在 to live后面）= The poor man has no house to live in（ in 后面没有which，必须放在 to live后面）. 【说明】(1) The poor man has no house in which he can live  中的  in which he can live 是定语从句，可以改为 which(that) he can live in (2)The poor man has no house to live in中的to live in 和The poor man has no house in which to live 中的 in which to live 是动词不定式短语作定语。（2）这几个句子的意思都是：“那个贫穷的男子没有房子住。”

       网友的句子He opened the north window from which to enjoy the distant hills.可以改写为：He opened the north window from which he can enjoy the distant hills.