Kannst du am Montag? – Nein, am Montag kann ich nicht.

The verb können belongs to the special group of verbs known as "modal verbs". These verbs are conjugated differently from the verbs that you already know.:

• Modal verbs often have a different vowel in the singular than in the plural.
• The form of the first- and third-person singular is the same: the verb has no ending.