The system of modal verbs is simpler than in English and interference may therefore cause mistakes such as the following:
The construction to be able to and to have to need particular care:
I can to do it.
You ust to work hard.
Yesterday, he must go home early.
I shall must phone him.
She will can do it.
Answers to questions with may and must cause diffiuclty:
May I come in?- No, you may not/can't. (meaning 'Please don't.')
Must I do it now?- No, you must not. (meaning, 'No, you needn't.')