The English modal verbs have different meanings and uses. Below you find the most common uses in cases where you might be in doubt about which verb to use.
Shall and must
Shall is used in contracts and legal documents to refer to obligations and duties.You should only use shall when the subject is a person or a body
The Owner shall be responsible for the supply.
Must also states a necessity and obligation, but in contrast to shall, it is only used in legal documents, when it is referring to an obligation stated in any other document than the current document:
The decisions must be made according to the Environment Act.
Click here to read more about the language use in legal documents.
Must is often used in directions of use and other text types to emphasise that it is important that something is done:
The ignition switch must be turned on before you can start the engine.
ShouldShould can be used as a weaker form of must. Where must expresses great confidence, should expresses less confidence and is understood more as a recommendation than an actual necessity:
You should get a colleague to read your document before you send it.
Can/could and may/mightWhen talking about the chance (possibility) that something will happen, or is happening, we use may, might or could, but not can:
We may/might hire more people next month.
Might is mostly used as a less definite or more hesitant form of may, suggesting a smaller chance:
He might attend the meeting.
When stating a permission, can and could are more common than may and might. May and might are more formal. Compare:
Can I look at your paper?
May I have a look at your newspaper?