Modals.

Can , Could, Must, Ought , To, Should, May, Might, are modals.
"Can you climb the tree? " Oh! yes! said the monkey.
"Can you climb the tree? "No! how can I?"said the donkey.
"Could you do that?" "Yes why not?"
"Could you spell the word wood?" "Yes I could".
"Must I read this now?" "Yes must".
"Can you come here please?" "Sorry! not now".
"We ought to obey our parents". "Are nt we?"
"You should read this before you leave ".
"You may go now " "The class is over".