I do aerobics. 我做有氧运动。

(1) 改为以 You 为主语的现在完成时疑问句
—> Have you ever done aerobics? 你做有氧运动了？

(2) 改为“我从没……” never
—> I've never done aerobics. 我从没做过有氧运动。

(3) 改为以 She 为主语的过去进行时
—> She was doing aerobics. （过去的某个时间段）她正在做有氧运动。

Someone is walking behind me. 有人走在我后面。

(1) 改为以 No one 为主语的句子

—> No one is walking behind me. 没人走在我后面。

(2) 改为以 Who 开头的疑问句

—> Who is walking behind me? 谁走在我后面？

(3) 改为以 Do you think 开头的疑问句

—> Do you think someone is walking behind me? 你觉得有人走在我后面吗？