Unless
Unless means if not and hence there is no need to add another not into the following clause.
- Incorrect: Unless you do not obey my instructions, you will be dismissed.
- Correct: Unless you obey my instructions, you will be dismissed.
- Correct: If you do not obey my instructions, you will be dismissed.
Lest
Lest means that…not and hence it is wrong to add another not in the following clause. Another point that needs to be noted is that the only auxiliary that can be used after lest is should.
- Incorrect: Take care lest you do not fall.
- Correct: Take care lest you fall.
- Correct: Take care lest you should fall.
That
That should not be used before the question words what, when, where, whether and why.
- Incorrect: He asked me that what was my name.
- Correct: He asked me what my name was.
No sooner…than
It is wrong to use when instead of than in these sentences.
- Incorrect: No sooner did the lion appear when he shot it down.
- Correct: No sooner did the lion appear than he shot it down.
If no sooner begins the sentence, it should be immediately followed by had or did.
- Incorrect: No sooner the lion appeared than he shot it down.
- Correct: No sooner did the lion appear than he shot it down.