which引导非限定性定语从句的用法

可以 The dictionary,which cost me 80 yuan,was lost .
My house,which I bought last year,has got a lovely garden.指代主句中的一个词I want to buy a gift，____ is expensive.为什么这道题不能填which呢？老师说要填one which，one做同位语，如果这样的话，老师的意思就是which指代主句的整体内容，填which不行，要有one，which来指代one。怎么回事呢？one=a gift泛指不是指前边提到的那个gift 。Yesterday I bought a gift for my mum, which cost me 20yuan.这里 which 指前面提到的给妈妈买的那个礼物 ，就可以用了。