Some Basic Rules for completing sentences:
Structure: Sub + verb + so + adj/adv + that + sub + cannot/ could not+ verb +other words.
Example: A. He is so clever that.......
He is so clever that he can answer all the questions.
The teacher spoke so quickly that......
The teacher spoke so quickly that we could not follow him.
That/so that (to express purpose)
Structure: Principal Clause + so that + sub may/might, can/could +verb +other words.
Example: A. We study so that......
We study so that we may pass.
They started early so that.......
They started early so that they could catch the train.
Note: In making negative sentences, we must use cannot/ could not, may not/might not.
Example: 1. I save for the future so that....... I save for the future so that I may not fall in trouble.
He closed the door so that......
He closed the door so that none could enter.
The meaning of 'lest' is ' for fear that'. 'Lest' gives negative sense. So, we cannot use 'no' or 'not' with it. But 'lest' must take should/might after it.
Structure: Principal Clause +lest + sub+ should/might + verb + other words.
Work hard lest...... Work hard lest you should fail
2.He started early lest....
He started early lest he might miss the train.
The house-owner treated the servant well lest......
The house-owner treated the servant lest he should leave him.