<p><strong>Auxiliary Verb</strong></p><ul><li>to be obliged or bound to by an imperative requirement.</li><li>to be under the necessity to; need to.</li><li>to be required or compelled to, as by the use or threat of force.</li><li>to be compelled to in order to fulfill some need or achieve an aim.</li><li>to be forced to, as by convention or the requirements of honesty.</li><li>to be or feel urged to; ought to.</li><li>to be reasonably expected to; is bound to.</li><li>to be inevitably certain to; be compelled by nature.</li></ul><p><strong>Verb (used without object)</strong></p><ul><li>to be obliged; be compelled.</li></ul><p><strong>Adjective</strong></p><ul><li>necessary; vital.</li></ul><p><strong>Noun</strong></p><ul><li>something necessary, vital, or required.</li></ul>