French error?

The card says: "On était morts de rire" but of course "On" is a singular pronoun and as far as I can tell the correct answer would be "On était mort de rire." Am I missing something?

Indeed, "on" replaces « nous » in colloquial french. "On" is not so impersonal in this case. It's obvious that it has been a massive joke attack where several pesonnes lost lives in here !
So you're right to correct in "nous étions mort de rire" / "on était mort de rire"
But we can consider "on est" as "c'est" are speaking language forms as in "on est les plus forts" / "c'est les plus drôles", and these ones need to be mentioned in the course.

Lol @MacDamien Thank you. So you are saying that in certain cases "on" may be grammatically considered a plural pronoun? I knew the implication of "on" is often plural but I didn't realize that gave us the option to pair it with plural adjectives or nouns. I wonder if Lingvist has made a mistake here or if they're trying to make us grammatically flexible like native speakers?

Thanks for raising this, I was confused by this question too. I found this here where someone says modern French grammarians accept plural adjectives applied to implicitly plural uses of 'on', giving the example on 'est jeunes',

I wish the card would explain this choice for confused learners. Thanks for the clarification.

You will never find "on est le plus fort", wich is a generalisation (impersonal) in the acception of "nous sommes les plus forts" wich is a concrete situation. We have no (not any (cup of tea)) alternative here.

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