Students usually understand the usage of the modals in present tense very quickly. However, when they start speaking about the past, they use the present tense or some non-existent forms, like “musted”.
Here I’ve tried to create a simple overview of all the modal verbs and their usage in present and past tenses. I hope you will find it at least a bit useful.

Have a look at this mind map. Notice that the modal verbs are in two colours. The white modal verbs are followed by an infinitive without to and the red modal verbs are followed by past participles. If you still have problems with the past participles please follow the link above or this one, which will take you to quizzes on past participles at our other site English Learning Magazine.
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Once you have understood the explanation, it is time to practise your knowledge in two games on Modal verbs. The first one is called En Garde and your task is to choose the correct modal verb and then hit your opponent. Modal verbs – En Garde The second game is called Penalty. Once again you should choose the correct modal verb and then try to score a goal. Will you succeed? Modal verbs – Penalty We have started a new site for learners of English. While this site contains short grammar explanations and games to practise the grammar, the new site www.englishlearningmagazine.com contains tests and quizzes to practise English in general. For example, there is a text on Easter and you have to answer comprehension questions and then reconstruct the text.
Or you could practise the names of the plants in a post which contains the video with the names of the plants and three tests. Moreover, you can listen to a song and complete its lyrics.
The whole site is mobile phones friendly (unlike this one, where the games play only on desktops).
Enjoy.