Did he have a picnic last weekend?why?为什么用did？什么时候用was开头？过去式啊，did是助动词，was是系动词， did提问的是过去发生的动作，接的是动词，如 Did you go(动词原形) to school yesterday? was直接提问的是名词、形容词等，如： Was he happy yesterday? 当然，如果是过去进行时或者过去将来时会用was，如： Was he sleeping when you came back yeasterday? Was he going to sing if he felt happy?你可以把句子还原成肯定句，was变成肯定句，也存在，did变成肯定句就不存在了，而是用动词的过去式代替。比如此题，变为肯定句是： He had a picnic last weekend. 而不是： He was have a picnic last weekend. 当然，如果说： He was going to have a picnic last weekend. 这种说法也可以，过去将来时。不过时间条件上应该更明显些!