Learn French: Le courtier aurait dû vendre l'action. - The broker should have sold the stock.

Aurait dû is the past conditional of the verb devoir (to have to / must). While the present conditional il devrait means "he should," putting it in the past conditional (il aurait dû) is how you express a reproach or a missed obligation in the past: "he should have."

Why is vendre in the infinitive here? In English, we use the past tense "sold."

In French, when you use a modal verb like devoir (even in a compound tense like the past conditional), the main action verb that follows it must stay in the infinitive. Think of aurait dû vendre literally as "would have had to sell."

Wait, doesn't l'action just mean "the action"?

It can, but it is a partial false friend! In a financial context, une action translates to "a stock" or "a share" of a company. Because this sentence mentions le courtier (the broker), we know it refers to the stock market.

Why does dû have a circumflex accent (the little hat) on it?

The accent on the masculine singular past participle dû is only there to distinguish it in writing from the word du (the contraction of de + le, meaning "some" or "of the"). The accent actually disappears in the feminine or plural forms (due, dus, dues).