Modalverben — the modal verbs

The six Modalverben — *können, müssen, wollen, dürfen, sollen, mögen — express ability, necessity, will, permission, duty and liking. They never appear alone: the structure is modal in position two, bare infinitive at the end — Ich muss heute lange arbeiten*. That frame is the sentence bracket at work.

Two habits worth building early: the dropped infinitive (Ich muss nach Hause = gehen is implied — idiomatic, not lazy), and Präteritum forms for the past (ich konnte, ich wollte, ich musste) — modals prefer simple past even in speech (why).