一般陈述句变一般疑问句结构是：be/do+主语+谓语+宾语
Is she going to buy a sweater for her mother?
陈述句是She is going to buy a sweater for her mother.（be going to do sht.是一般将来时态,表示计划打算做某事）,谓语是IS.所以变一般疑问句时是将IS提前后接一般疑问句式）
Does she plan to buy a sweater for her mother?
陈述句是She plan to buy a sweater for her mother.（plan to do sht.计划做某事.因为plan to本身含有将来时态.这个句子是一般现在时表示将来.因为plan to是实义动词.所以变成一般以为句式是助动词do引导一般疑问句式.
综上.动词不一样疑问句式就稍有变化.