OUT 的语气比较粗鲁
OFF 的语气比较平和，普通人都这么说
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？。
