set off:出发,侧重于去某个地方 例句与用法:If you want to catch that train we'd better set off for the station immediately.你要是想赶上那班火车,咱们就最好马上动身去火车站.They've set off on a journey round the world.他们已经开始环游世界.set out:出发,开始,侧重于开始做某事,还有规划,展现的意思 例句与用法:They succeeded in what they set out to do.他们打算做的事已经做成了.set about:开始,着手,与set out意思相近,但set out后加不定式,即set out to do sth,而set about后加 ing形式,即set about doing sth.例句与用法:1.You must set about your work at once.你必须立即开始工作.2.Do you know how to set about going on this work?你知道怎样着手进行这项工作吗?