Modal verbs

Today our topic was Modal Verbs and have learnt that these also are called auxiliary verbs and express different meanings like in the case of Can: it means or express ability, request or permission.
For example: I can play the guitar. ( It is an ability because no all of the people can do it)
* Can I help you? ( I am offering my help to someone)
Also we should know that the modal verb goes before other verbs and they don't have -s on the third person singular and the other verb that goes next to the modal verb goes in its own form. It does not go without ed, ing or the particle to
Ex: She can play the piano (correct)
She cans play the piano (incorrect)
She can plays the piano (incorrect)