4.104 no inflections

Modals do not inflect. This means there is no '-s' form in the third person singular, and there are no '-ing' or '-ed' forms.

• There's nothing I can do about it.
• I am sure he can do it.
• I must leave fairly soon.
• She insisted that Jim must leave.

'Could' is sometimes thought to be the past tense of 'can'. This is discussed in paragraphs 4.113 and 4.114.