<p><strong>Auxiliary Verb</strong></p><ul><li>A simple past tense of can.</li><li>(Used to express possibility):<ul><li>I wonder who that could be at the door.</li><li>That couldn't be true.</li></ul></li><li>(Used to express conditional possibility or ability):<ul><li>You could do it if you tried.</li></ul></li><li>(Used in making polite requests):<ul><li>Could you open the door for me, please?</li></ul></li><li>(Used in asking for permission):<ul><li>Could I borrow your pen?</li></ul></li><li>(Used in offering suggestions or advice):<ul><li>You could write and ask for more information.</li><li>You could at least have called me.</li></ul></li></ul>