They tend to be irregular verbs and you can also inverse the sentence with them. Here is an example of the inversion, "He can go" becomes "Can he go". In most cases they are followed be the infinitive of another verb. Here is a brief list of examples of modal verbs: can, could, may, might, will, must, would, and shall. A lot of the time these verbs will be used in probability, ability, permission, or advice.

Ability example "You can speak two languages."

Probability example "It is raining, so it must be wet outside."

Permission example "Can I drive your car."

Advice example "If you want to learn a language you must study"

Can and must are the modal verbs in the above sentences. I hope this helps out a bit. I am sure Joseph's booklet on it will be of even greater value. Good luck in your English study!

A great explanation with exercises to practise:

http://www.englishpage.com/modals/modalintro.html

They are verbs which don´t act like other verbs,  they have different rules. They are very simple, I have written a little booklet about them to help my students, if you send me your Email Ill pass a copy to you

Andrei,

If you don't want to help, then you don't need to comment

Eu também tinha esta dúvida, Amanda. Veja isto:

https://www.youtube.com/watch?v=2NCBo8o2Wmk