As we’ve noted in previous lessons, accent marks are very important in French. Their presence or absence can completely change the meaning of a word, as in cote, côte, and côté or des, dés, and dès. In this lesson, we’ll investigate a more straightforward but no less significant distinction, between du and dû. 

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You may already know that in French de + le ("of" + "the") is always contracted into du. That’s why, in their introduction to their video on springtime trends (or trends of the springtime), Fanny and Corinne say tendances du printemps:

On va vous parler des tendances du printemps.

We're going to tell you about some springtime trends.

Caption 3, Fanny & Corrine parlent de la mode - Tendances du printemps

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Printemps is masculine, so, to put it mathematically: de + le printemps = du printemps. Note that, in the title, Fanny and Corinne parlent de la mode (talk about fashion). De + la can appear together in French, so no contraction is necessary there. You can find out more about these rules on this page.

When you put a circumflex on du, its pronunciation doesn’t change, but it’s no longer a contraction of de + le. Dû is the past participle of the verb devoir, which means “to have to” or “to owe.” So why does dû require a circumflex? For no other reason than to distinguish it from du! Though the circumflex is only used to distinguish meaning in this case, it can serve some other purposes as well, which you can learn more about here.

Here’s an example of dû used as a past participle, from a video about an electric sporting boat: 

Donc, on a dû utiliser deux moteurs.

So we had to use two motors.

Caption 25, Bateau sport 100% électrique - Le Nautique 196 E

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Dû can also be used as an adjective, in which case it means “due,” as in the expression “due to” (dû à): 

Peut-être que c'est aussi dû au fait que ma mère aimait beaucoup chanter.

Maybe it's also due to the fact that my mother liked very much to sing.

Caption 16, Mai Lingani - Mai et Burkina Electric

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Dû is the masculine singular form of the adjective, but note that the circumflex disappears in every other form: the feminine singular (due), the masculine plural (dus), and the feminine plural (dues). Remember: in this case, the circumflex is only there to prevent confusion with du.

In this caption from a video on AIDS, dû modifies the singular feminine noun banalisation, so it becomes due: 

Une banalisation qui est due d'ailleurs à la trithérapie.

A trivialization which, besides, is due to the tritherapy.

Captions 3-4, Le Journal - Le sida

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Finally, dû can be used as a noun (un dû) to mean “a due,” or something that one is owed: 

Je lui paierai son dû. 
I will pay him his due. 

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We hope that we have duly (dûment) demonstrated how much of a difference one little accent mark can make!