Why not?

Pour ceux qui n'ont pas eu le temps de tout noter:

4 ) He sang my favourite song.

My favourite song was sung (by him).

5) He had won the elections before the first round.

The elections had been won by him before the first round.

6) They haven’t displayed the polls before the results.

The polls haven’t been displayed by them.

7) She slammed the door.

The door was slammed by her.

8) She will get the best mark in the class.

The best mark in the class will be got(ten) by her.

9) He can’t tell the truth.

The truth can’t be told by him.

10) You must take care of your little brother.

Your little brother must be taken care of.

Pour les autres, voici quelques exercices en ligne:

http://sitasima.pagesperso-orange.fr/passif/construction.htm

(Lisez la leçon puis faites les exercices 1, 2 et 3)

http://michel.barbot.pagesperso-orange.fr/hotpot/voix_passive/vp0.htm

http://englishonline.free.fr/GrammarAndHelp/Passive/TransformIntoPassive01.jcl.htm

Keep cool!