【1】sudden:指事情的发生可能在意料之中，但来得太快是意料之外，侧重强调事情发生动作的突然性。

His decision to quit from the company was very sudden.  

他决定退出公司很突然。

He gave me a fright by bursting into a sudden laughter.  

他突然大笑起来，吓了我一跳。

I was struck by the sudden realization that I would probably never see her again.   

我突然意识到可能再也见不着她了

I wonder what's behind his sudden change of the plan.

我真不知道他突然改变计划的原因是什么.

My plans were completely deranged by his sudden arrival.

他的突然来到把我所有的计划全部搞乱了.

 【2】abrupt:指事情发生来的太突然，完全出乎意料，需急速调整以适应突然的变。化，常暗示突发事情会带来不愉快的、令人惊慌的结果。

The train came to an abrupt stop, making many passengers fall off the seats.  

火车突然停了下来,使许多乘客从座位上摔下来。

The driver made an abrupt turn to avoid hitting another car.  

司机一个急转弯,避免撞了与另一辆车相撞。

Most strokes are caused by an abrupt blockage of arteries leading to the 

brain.   

大多数中风都是脑部动脉突然阻断引起的。

I don't know the cause of his abrupt leave.