which one i should choose.→i haven't decided ().
i don't know when i will come back【when to come back】请问【i】为什么去掉?
使用特殊疑问词+动词不定式的结构不需要使用主语。另外如果加 I,也实在没地方加啊,除非加个 动词 不过加个动词又变成复合句了 。就不是简单句了