4º ESO. Unit 8. Modal Verbs

I've found this pdf file about Modal Verbs on the Internet that may be of help to you. All the theory is in Spanish and a lot of examples in English.
This presentation by Anabel Ponce will also be useful. Finally, try these exercises using the link with the mouse icon. This other website http://www.englishpage.com/modals/interactivemodal1.htm  is recommended for advanced students.