I can't decide which dress to buy 为什么不可以说是I can't

网友回答

which dress to buy是动词不定式短语,要由特殊疑问词来引导,特殊疑问词要放在短语的前面.
其结构就是：特殊疑问词+to do.又如：
I don't know whom to ask.
I wonder when to leave.
Can you tell me how to get there?
======以下答案可供参考======
供参考答案1:
这句话是宾语从句，在主谓之后先要放一个引导词，这里就是which。
这个引导词引导从句，再接从句的主谓宾。