"on" is a personal subject pronoun (third person singular, replaceable by "il" or "quelqu'un"). "ont" is the verb avoir conjugated in the third person plural (replaceable by "avaient").
Replace with "avaient". If it works, write "ont". If not, write "on".
Examples
On mange bien ici.
Key word(s): on
"Avaient mange bien ici" does not work. "Il mange bien ici" works.
Ils ont fini.
Key word(s): ont
"Ils avaient fini" works.
On a trouvé la solution.
Key word(s): on
"Il a trouvé" works, so this is the subject pronoun.
Elles ont raison.
Key word(s): ont
"Elles avaient raison" works.
On dit qu'ils ont gagné.
Key word(s): on, ont
"On" = subject pronoun. "Ont" = the verb avoir (ils avaient gagné).
Common mistakes
Writing "ils on fini" (using the pronoun instead of the verb).
Writing "ont mange bien" (using the verb instead of the pronoun).
Getting confused when "on" is followed by "a" ("on a" vs "ont").
Test yourself
___ va au cinéma.
"Avaient va au cinéma" does not work, so this is the pronoun.
Les enfants ___ joué.
"Les enfants avaient joué" works.
___ dit qu'ils ___ tort.
"On" = pronoun, "ont" = the verb avoir (ils avaient tort).
Tips to remember
"on" is always the subject of the verb that follows.
"ont" is always preceded by a plural subject (ils, elles, les enfants...).
Watch out: "on a" and "ont" sound the same but are very different!
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