Question

“我们正在骑车，这时听到有人正在大喊”用英语怎么说

（We ? we heard somebody shouting form behind）这个句型的

Answer 1

were riding bikes when

Answer 2

We were riding when we heard sombody shouting from behind.
这里要表达的是过去正在做的事情，所以用过去进行时。