they are walking over the bridge为什么walk要加ing?

they are walking over the bridge为什么walk要加ing? They are walking over the bridge. they are walking over the bridge为什么不是across the bridge? they are walking over the bridge 为什么不用'on the bridge' they are walking over the bridge 为什么不用on the bridge they are walking over the bridge其中over的问题 They are WalKing over the bridge.(划线提问)They They are walking over the bridge. 主语They 谓语 are walking 那么ov who are walking over the bridge? they are walking over the bridge中的over改成across有没有问题? Walking is good exercise,为什么walk要加ing? 会英语的大师来帮帮忙,They are walking over the bridge.中ov