Use may not and might not to express the possibility that something will not happen.

He has good intentions, but he might not solve the problems.

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Use couldnt to express the idea that something is impossible.

A: Why dont you cut class tomorrow?B: I couldnt do that. I have to give a presentation .*

Questions about possibility usually do not use may, might, or could. Instead, they use the future (will, be going to, the present progressive) or phrases such as Do you think? or Is it possible that?

A:Do you think it will rain tomorrow too?B: It could stop tonight.

The answers to these questions often use may, might or could.*

When we are almost 100% certain, we use must, have to, or have got to to state affirmative conclusions.

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FACT CONCLUSIONWilson has only one clerk.His shop must be quite smallWilson applied for a job.He has to need money.They pay men for debating with the president.Its got to be a joke.

When we are less certain about our conclusion, we use may, might, or could to express that something is possible.

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FACT CONCLUSIONHis hand is swollen.He may write a lot.Watson knows a lot about medicine.He might be a doctor.Vincent knows a lot about cameras.He could be a photographer.

To express negative conclusions:Use cant and couldnt when you are almost 100 percent certain that something is impossible.He cant be dead! I just saw him!Use must not when you are slightly less certain.He must not have enough money. He never buys new clothes.*

Use may not and might not when you are even less certain.He may not know about the plan. His boss doesnt tell him everything.Do not use have to and have got to to draw negative conclusions.CORRECT: It cant be true!INCORRECT: It doesnt have to be true!

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utpl*utplutplWe usually do not contract might not, and we never contract may not.utpl*utplWe often use have got to instead of have to in informal speech and writing. We usually contract have or has.utpl*