先行词被 the only one of 修饰时关系代词用that ,可就有用who的时候,另先行词为不定代词时怎也会用who

题目

先行词被 the only one of 修饰时关系代词用that ,可就有用who的时候,另先行词为不定代词时怎也会用who
Tom was the only one of the boys who was on time.
汤姆是那些男孩中唯一一个准时到的.当先行词有the only 时,的确是用that的,that本来就是既可以指人也可以指物的呀?为何还要用who?所以,我也搞不懂啊?
我还碰到过类似的.我们GO for it --人教版的九年级的 Unit 4 Section A中3a里还有先行词为不定代词时用who的情况,如：If I were you ,I'd talk to someone who looks friendly.我也不解,

答案

首先第二个句子If i were you ,I'd talk to someone who looks friendly.这句话中为什么不用that 因为who 要做look的主语,而that 不能充当主语成分,所以这边用who 而不用that.同理,第一个句子也是这样,who 是做was的...