When they were walking through the forest, a bear came at them.主句是a bear came at them。When they were walking through the forest是时间状语，正当他们走过森林时。
Be careful， the train is coming.当心，火车正在向这边开过来。通常翻译为：火车要来了。按中文意思是 will come，但不符合英语习惯。例如：我要走了。I'm going.而不是I will go.
内容全部来源于网络收集，如有侵权，请联系网站删除：QQ：24596024