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是decide的宾语.
句意:我不能决定要买什么.
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是decide的宾语.
句意:我不能决定要买什么.