Divide the class into groups or teams of three or four. Ask the class a series of general knowledge questions in the passive voice. For example, "Who was the Taj Mahal built by?" "Who was the first soccer World Cup won by?" "In which country are the Pampas found?" Check answers as a class and award each team one point for each correct answer. Students now work in their teams to write five general knowledge quiz questions of their own in the passive voice. Circulate and give help where necessary. When each team has finished, team members ask their questions to the class. Award each team one point for each correct answer.
Show the class, on an overhead projector, a picture, but ensure the overhead projector is completely out of focus. Ask students to speculate on what the picture could be using modals of possibility and probability, for example, "It could be a tower" or "Maybe it is a house." Slowly bring the picture into focus asking students to continue speculating. For example, "It might be some kind of animal." "It must be a cat." "It is a cat!" Repeat with new pictures for 10 minutes or until the students' interest starts to wane.
Divide the class into pairs. Give each pair a set of cards on which you have written questions appropriate to the level of your class using modal verbs. For example, "What could you do as a child that you can't do know?" "What should I do to improve my English?" "Why might you run out of the class screaming?" Students ask and answer the questions in pairs. Circulate and make notes of any mistakes. After 10 minutes or when each group has finished, stop the activity and ask each student to write five more questions using modal verbs. Again, arrange the class into pairs to ask and answer their questions. When each pair has finished write any examples of bad language you heard on the board for the class to correct.
Tell the students that when you arrived at the school today and entered your office you found the table upside down, a window broken and a cat in the corner of the room. Divide the class into groups of three or four and ask them to write as many sentences as possible in 10 minutes using modals to describe what happened. For example, "Maybe the cat broke the window trying to get out of the room." "You might have been robbed." " You must have gotten really drunk last night, stolen a cat and wrecked your room!" Circulate as the students work, giving help and encouragement where necessary. After 10 minutes stop the activity and ask each group to read its sentences to the class.