I can't decide what to buy.解析：这句话是不定式短语代替宾语从句.题中what to do也可以还原成宾语从句：I can't decide what I should buy.原句中,I 是主语,can't 情态动词,decide连在一起共同做谓语,what to buy...不定式短语中what做什么成分呢？原句应该是I can't decide to buy what. what为什么要提前呢？这个题目中可以说what做的是buy的宾语，因为还原成从句就是buy的宾语。宾语从句中疑问词需要提前，紧跟在及物动词（此处为decide）之后。你说what to buy是decide的不定式做宾语就可以了。非要拆开来看就像我上面说的就行了。Decide to do sth. What就是sth，因为是疑问代词所以提前了，对吗？what to do是sth，因为what是疑问词，所以就得提前了。