You've already met the idea of modal verbs; this lesson covers all six of them and locks in the sentence-building pattern you'll need in almost every conversation: can, must, want, like, may, should.
Each of the six — können, müssen, wollen, mögen, dürfen, sollen — expresses a different stance toward another action. Können is physical or mental ability ("I know how"). Müssen is strong, unavoidable necessity. Wollen is personal will. Mögen, in its polite form möchte, is desire or preference. Dürfen is permission granted from outside. Sollen is advice or an assignment from someone else. The difference between müssen and sollen matters: müssen means there's really no choice, as with medical or legal necessity, while sollen is closer to someone else's recommendation — the decision still rests somewhat with you.
Structurally, the rule holds: the modal verb conjugates and takes second position; the main verb stays unconjugated, in the infinitive, at the end. One detail saves many mistakes: for modal verbs, the ich and er/sie/es forms are identical, without the usual -t ending regular verbs take. So it's ich kann and er kann (never er kannt), likewise ich muss and sie muss — an exception unique to modal verbs.
Sometimes the main verb can be dropped entirely when the meaning is obvious from context, especially with verbs of motion or knowledge: Ich kann Deutsch implicitly means "I can speak German," even without sprechen. In yes/no questions, the modal verb itself jumps to the front: Kannst du Arabisch sprechen? — but the main verb sprechen still stays at the very end, even in a question.
The most common mistake is conjugating the main verb too, producing Ich kann spreche instead of Ich kann sprechen. Only one verb in the sentence ever conjugates — the modal — and the second verb always stays in its plain, unconjugated form.