I have already published a post on possessive case. But when I tried to teach it again, I found out that it was not clear enough for the group I am teaching now. So I created a new infographic and several new interactive exercises in which students can practise the grammar. And my new students understand it now nearly perfectly. I hope you will find these materials useful too.
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Have a look at this infographic and see how we form the possessive case in English.

To cut the explanation short, add the apostrophe+s to a singular noun and add only the apostrophe to a regular plural noun. If the noun has an irregular plural add the apostrophe+s. If you feel that you understand the grammar, it is high time to practise it in the following exercises.
The following quiz is in HTML5, so it will work on all mobile devices. Thus your students can practise the grammar anywhere they like. If they pass the quiz they will be given a chance to play a game as a reward.

If you feel that the quiz is too small, click the button below and play the game on the full screen.

Possessive case – quiz full screen

If you are looking for more practise I recommend British Council page on possessive case.

If you like the quiz above and you would like to share it on your blog or use it in a classroom without an internet connection, you can do this. You can download all the files here and upload them to your site just unpack the files and use them in the classroom:
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[showmyadsa] Possessive case_quiz (Web)