Khan Academy
Khan Academy: Modal Verbs
The modal verbs of English are really powerful; you can use them to ask for permission, give advice, determine how likely something is, and many other things besides! Test your knowledge of the modals!
PBS
Pbs Learning Media: Modal Auxiliary Verbs
Modal auxiliary verbs indicate expressions of belief, attitudes, likelihood, permission, or obligation. They inform the function of the main verb, and color the verb on a scale ranging from possibility to necessity. Video (0:47)