lest：以防;从句中的谓语动词必须用should+动词原形;

eg: He wrote the name down for fear that(lest) he should forget it.

He took the umbrella with him lest it should rain.

in case：以防;与lest不同，从句里的时态不做特殊变化;

eg：Take the umbrella with you in case it rains.

for feat that：和lest保持一致;

eg：A variety of activities should be organized so that participants can remain active as long as they want and then go on to something else without feeling guilty and without letting the other participants down.

let…down…：让……感到气馁;