He may be at school now,isn't he?
He may swam at the swimming pool last night,didn't he?
1:第二句的动词时态错误
2:变反义疑问句,首先要看动词.这两句话may只是疑惑人的,真正的动词是be和swim,所以你就根据动词来变换否定式,然后再加主语,就可以了.