Learn French: L'investissement était risqué, il aurait dû garder son argent. - The investment was risky, he should have kept his money.

Aurait dû is the past conditional of devoir (to have to / must). While the present conditional (il devrait) translates to "he should," the past conditional means "he should have." It is formed with the conditional of the auxiliary avoir (aurait) plus the past participle of devoir (dû). In this sentence, it perfectly expresses a regret or reproach about a past mistake.

Why is garder in the infinitive form when we are talking about the past?

In French, when verbs follow each other, the second verb is almost always in the infinitive (the unconjugated dictionary form). Here, garder directly follows the conjugated structure aurait dû. So, while English says "should have kept" using a past participle, French says "should have to keep," leaving garder unconjugated.

Why use the imperfect était instead of the passé composé a été for "was risky"?

The imperfect tense (était) is used here because it describes a past state, condition, or characteristic. It simply sets the scene by describing the inherent riskiness of the investment, rather than describing a specific, completed action with a clear start and end.

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