Rewrite the sentences by replacing the underlined expressions with the correct modal verb (couldn't, can, need, needn't, ought to, must). Make the necessary changes.

1. Richard can solve maths problems very quickly.
2. You needn't stay in the exam hall after you have completed the test.
3. You should seek help if you are having problems understanding something.
4. Nia mustn't have been nasty to the other children.
5. You must have a computer password.
6. Students must be on time for class.