Modal verbs

Use modal verbs to express ideas such as ability, uncertainty, necessity and permission. Use modal verbs sparingly and at your discretion, and always use the correct verb for the instruction.

• Can—relates to present ability and general possibility.
• Could—relates to possibility and suggestion.
• May—relates to probability and permission. It also means that an item is optional.
• Need—relates to the absence of obligation. In technical documentation, this verb is used only as a synonym for the verb want.
• Must—relates to personal obligation. It means that the definition is an absolute requirement of the specification.
• Have to—relates to impersonal obligation. Don't use it as a synonym for the verb must.
• Should—relates to expectation and recommendation. Don't use it as a synonym for the verb must.