catch sb doing sth或catch sb’s doing sth表示：当场发现或发觉某人做某事
请看：
I caught her smoking in the bathroom.
我撞见她在盥洗室里抽烟。
You wouldn't catch me working on a Sunday!
你绝对不会看到我在星期日工作！
She caught herself wondering whether she had made a mistake.
她发觉自己在怀疑是否犯了错误。
I caught myself feeling almost sorry for poor Mr Laurence.
我突然发觉自己几乎同情起可怜的劳伦斯先生来了。
He caught a youth breaking into a car.
他撞见一个青年正在撬车。
He wouldn't be caught dead going to a club with his mother.
他死也不愿意跟母亲一起去俱乐部。
另外：catch a bus/flight/plane/train搭乘公共汽车/航班/飞机/火车
指及时(赶上)登上飞机、火车、公共汽车等。
You'll have to hurry if you want to catch that train.
如果你想赶上那趟火车就得抓紧时间了。
He got up very early so as to catch the first train.
他起得很早，以便赶上头班火车。
He suddenly discovered that it was too late to catch the last bus.
他突然发现时间很晚，赶不上末班公共汽车了。
She was about to leave the office to catch a flight to London.
她正要离开办公室去赶飞往伦敦的一班飞机。
We were in plenty of time for Anthony to catch the ferry.
我们时间充裕得很，足够安东尼赶上渡船。
I had to run like mad to catch the bus.
为了赶上公共汽车，我不得不拼命跑。