He is busy at work.work可以加ing吗 有什么区别

He is busy at work.work可以加ing吗 有什么区别 work at work under 有什么区别 work on may letter和work at my letter都可以吗,有什么区别 at work,in work,on work 有什么区别? He is busy at work.能否改为he is busy working work on,work at的区别、联系,可以相同吗,后面一般加什么,详细一点好, 他正在工作 翻译成英语：He is at now.Work要加s或ing还是原型? work at,work in ,work for ,work out有什么区别 at work 和 in work有什么区别? work from home跟work at home 有什么区别 work on 和 work at 有什么区别? work on 和work at 和 in 有什么区别?