In the example “On est partis.” partis is made plural even though on is considered singular. Is this correct?

Question

Accepted Answer

Hi Heather, 
'On' in this case is 'we', so the past participles agrees in plurality, a bit quirky but correct...

Answer

Hi Jinn,
On is like il or elle in terms of conjugating the verb.
Take a look at the following lesson for examples  
https://french.kwiziq.com/revision/grammar/subject-pronouns-nous-versus-on
and in particular the green box :
On is a more informal we, used predominantly in speech or casual writing (in emails to your friends for example). This is also a singular pronoun which is followed by the same form of the verb as singular il or elle. 
Hope this helps!

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But ètre is still singular, 'On est partis tôt.'? Is it correct when 'On' is 'we', the past participle is in plural form, but ètre is in third person singular form.

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Vous êtes parti and vous êtes partis are both accepted as vous can be singular  or plural. The form of être doesn’t change, just the past participle. We just need to think of “on” the same way, it can have a singular or plural meaning, still followed by “est”, but a past participle that changes with plurality.

Answer

On est parti is CORRECT