Class 19-2: Expressing Degrees of Certainty in the Negative

If you are 100% sure the topic you are discussing is not correct, use is not or isn’t. Remember, is is used for a statement of fact.

Examples:

1. Tom isn’t sad.

2. Mary isn’t slow, she just won the race.

If you are 90% sure the topic you are discussing is not correct, use could not be or couldn’t be.

Couldn’t be is used when you believe the possibility of the topic being true is very small.

Couldn’t be is also used if you are sure the topic is not true but you have no evidence to prove your opinion.

1. Seth couldn’t be blind, he just read a magazine.

2. Seth couldn’t be starving, he just ate lunch.

If you are 80% sure the topic you are discussing is not correct, use must not be.

Similar to couldn’t be, you have no evidence to support your opinion. You have a strong belief.

The difference between must not be and couldn’t be is that you are thinking more logically when using must be.

Question: He just lost the race. I thought he was fast?

Answer: He must not be.

If you are 60% sure the topic you are discussing is not correct, use may not be or might not be.

You are discussing suspicious or uncertain thoughts about the topic.

Question: I thought she was going to Harvard University?

Answer: She may not be.