<p>I know "but" is usually a conjunction. In one practice test I took (Barron's PSAT), it said "but" can be at times used as a preposition. </p>
<p>Could someone clarify this distinction? </p>
<p>It can mean “except.” </p>
<ul>
<li>I ate nothing but artichokes for dinner last night.</li>
<li>All but one of the artichokes were infested with deadly parasites.</li>
<li>Usually I will accept nothing but the finest from my artichoke dealer.</li>
<li>Do you promise to tell the truth, the whole truth, and nothing but the truth?</li>
</ul>