Which one?

What’s the difference between on and l’on? In a nutshell, on is sometimes preceded by l’ for reasons of euphony. This usage is formal and much more common in written rather than spoken French.

First things first: on is the impersonal subject pronoun and l’ here is a definite article (as opposed to a direct object).

at the beginning of a sentence is very old-fashioned and formal, a relic ofwas originally the subject case formeant. French lost the subject case around the 17th century, andbecame the pronoun we know and love today.

L’on is never required, but is preferred when formality and elegance are desired, for euphony:

1) At the beginning of a clause

Because it just sounds better (see also un vs l’un):

Par exemple…

L’on est obligé d’enlever les chaussures. People are required to take off their shoes. Demain, l’on va étudier un poème. Tomorrow, we’re going to study a poem.

2) To avoid hiatus

After the monosyllabic, vowel-sound-ending words et, ou, où, qui, quoi, and si, l’on is preferable to on so that there’s no hiatus (two vowel sounds side by side).

Par exemple…

Je ne sais pas de quoi l’on parle. I don’t know what people are talking about. C’est la maison où l’on habite. That’s the house where we live.

3) To avoid con

After lorsque, puisque, and que, using l’on avoids the contraction and thus pronouncing (even silently inside your head) what sounds like the offensive word con.

Par exemple…

J’espère que l’on arrivera à l’heure. I hope we arrive on time. Puisque l’on parle de dîner … Speaking of dinner …

1) After dont

However,is required in two places:

The liaison in dont on sounds better than dont l’on .

Par exemple…

L’homme dont on parle … The man we’re talking about … Je ne sais pas ce dont on a besoin. I don’t know what we need.

2) In front of l

L’on followed by a word beginning with the letter L would sound odd.

Par exemple…

Si on laisse nos affaires ici … If we leave our things here … Savez-vous où on loue les voitures ? Do you know where we rent the cars?

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