What are modal verbs? They are auxiliary verbs which modify or change a bit the meaning of the main verb.

FREE ONLINE MODAL VERBS EXERCISES
online and printable exercises with answers
plus notes, video and audio lessons

by Bob Wilson at autoenglish.org

ONLINE EXERCISE PRINTABLE EXERCISE VIDEO & AUDIO LESSON GRAMMAR NOTES
Can, could, may & might - mixed use    
First Certificate auxiliaries & modals exercise      
Have something done
Key word transformations for FCE    
Modal verbs of ability - can, could & be able to
Modal verbs of advice - could, should & would
Modal verbs of deduction - must have, can't have
Modal verbs of necessity - need, needn't & should  
Modal verbs of obligation - must, have to & should
Modal verbs of obligation - don't have to & mustn't
Modal verbs of permission - Make, let & allowed to  
Modal verbs of possibility/probability- can't be, might be & must be
Perfect modal verbs - might have, should have
Questions with auxiliary & modal verbs      
Semi-modal verbs of advice and preference - had better and would rather
Semi-modal verbs - used to, be/get used to
Semi-modal verbs - had better & be supposed to