What's "modal" about this verb?

The small class of words known as "modal verbs" is made up of verbs that do not denote an action (as is normally the case, e.g. "tanzen," "to dance"), but rather the way something else is experienced - that is, they express modality. So, you can indicate someone's relation to an action using modal verbs: "Sie mag tanzen" ("She likes to dance"), or "Sie muss tanzen" ("She must dance").

Modals exist in English too, and they are most often used with another verb. When this happens in German, that other verb appears at the end of the clause in its infinitive form. Modals are not typically used in the Perfekt tense, so you only have to worry about knowing the simple past (Imperfekt) form. For more information, see the examples for particular verbs or these topics in Grimm Grammar: Modals in present tense, Modals in past tense (Imperfekt).