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?
If "on" is used to replace "nous", i.e., referring to a group of people of which the speaker is also a part, the adjective must agree with the noun. Hence, in this case "on était morts de rire" is correct. However, if "on" is used to refer to mankind (e.g., En 1969, on est allé sur la lune), or used in statements describing rules (e.g., On doit être reconnaissant de sa vie), the adjective takes the singular form.
Thank you all for this interesting thread!
Indeed, as many have already pointed out, if 'on' represents 'nous', the adjective and past participle agree to it in number (and gender, if 'nous' is 100% feminine).
So you do indeed say 'On était morts de rire' (the verb is always in third person singular) or 'On est partis tard'.
@Ry That's a good idea! I'll add a little explanation about it after the card.