were sb doing还是were sb did sth?就像这个句子：Were they planting a big tree in the garden?还是Were they planted a big tree in the garden?那个...还有一个We must buy enough food for the trip.怎样改一般疑问句？

Were they planting a big tree in the garden?
must we buy enough food for the trip?

第一个

第一个
be动词加动词ing形式表示正在进行
没有be did
只有be done 表示被动

Were they planting a big tree in the garden?

Were they planting a big tree in the garden?
be 动词后面跟ing形式的
Must we buy enough food for the trip?

选择前者．因为是询问一个过去的 在种树的 状态
后者想要询问的话应该 以did开头
因为是针对后面种树的动作
第二个问题
改为
do we have to buy enough food for the trip?

Were they planting a big tree in the garden是对的
Were they planted a big tree in the garden还原是 they were planted a big tree in the garden.不能用被动，是错的