Agreement with pronoun "on"
The agreement with the pronoun "on" largely depends on what it refers to.
When on = everybody, pleople in general, "on" is replacing pronoun "il" and it is the third singular person. This usage of "on" is very formal.
- Dans ce pays, on dîne à 20h. - On dit que de tous les peuples de la Gaule les Belges sont les plus braves.
We can also use pronoun "on" instead of pronoun "nous". The agreement is then made as if one were the first person plural with the past participles. This use of "on" is mainly done orally. We greatly prefer to use the "on" instead of the "nous" in the oral.
- On est allés à la plage hier, j'en suis encore fatigué. - On est rentrés tard cette nuit.
In a sense close to irony, one can also put "on" in the place of "tu" and "vous". "On" then replaces a second person.
- C'est à cette heure-ci qu'on arrive. - Alors, on n'a pas fait aujourd'hui ?
Finally, the picture wouldn't be complete if "on" didn't replace the "je". The chord is then like the first person.
- Deux secondes, on arrive.
- On fait ce qu'on peut.
In all the conjugations of "on", the mark of the person is made only on the past participle. The verb or auxiliary is always in the third person singular.