On is a French pronoun. Its usage is strange and difficult.

On derives from the Latin homo (human being) through the Ancient French ome. The same root produced the modern word homme (man). on used to be a noun, and was used with an article: l'on meant the man, someone. The article is still used sometimes in modern French : que l'on me prévienne lorsqu'elle sera arrivée (someone tell me when she's here.).

Nowadays, on is used with the following meanings:

  • as a variant of we in informal speech. When talking with friends, a Frenchman will rarely use nous (we), unless the several possible meanings of on conflict in that context.
On a gaaaagnéééé ! (we've wooooon! Yelled by thousands of rugby or football fans)
On a mangé notre pain blanc (we have eaten our white bread, i.e the good part is over and the difficult part begins now)
  • as a variant of he, she or they, when you don't want or cannot name the agent. Sometimes you don't even mention whether there is one or several agents: On m'a dit (I've been told by one or several people), on m'a fait beaucoup de mal (I've been hurt a lot). It can be used to translate the English pronoun one in sentences such as one cannot use these stairs. The level of language is more formal then when using on for we. This on is often used by paranoids and politicians: travailleurs, on vous ment, on vous exploite (workers, they lie to you, they exploit you). Using on is often a sign that the speaker cannot prove what he says.
A Frenchman would probably even understand the following sentence, where "on" is used with two different meanings: On nous a fait du mal, alors on s'en va (someone hurt us, so we're leaving).
  • on may be vague to the point of meaning both we and they at the same time. On mange bien ici means the clients eat well in this restaurant, and I know it because I have already eaten here.
  • sometimes it means you, when talking to an inferior being (your pet, a baby, or an old and ill person): alors, on a fini sa soupe ? (so, have you finished your soup?). Don't speak like that to your grandmother.
  • in the following beautiful phrase: le qu'en-dira-t-on, which means what they/the people say (about you, about it). Je me fous du qu'en-dira-t-on: I don't care about what 'they' say.

Because on can replace several different pronouns, its usage raises difficult grammatical agreement issues. on is a singular 3rd-person male pronoun, just like il. But in some circumstances the agreement will be done with the meaning, which may be plural (we, they) or feminine.

  • Agree a non-compound verb with the 3rd singular person: on part (never on partent or on partons).
  • Use the plural for direct object complements if the meaning is we or they: on est les plus forts (est is singular while les plus forts is plural. Nobody will say on est le plus fort.), or on est les plus fortes if you are all women. Note that feminine agreement of on is very unlikely when on means they, because in that case on designates an indeterminate quantity of people whose sex we probably don't know.
  • The participates and adjectives are usually plural or feminine when on points to a group of people or to women. You will probably write on est partis (we have left) instead of on est parti and on est belles (we are beautiful) instead of on est beau if you work for Elite.

A rule of thumb about agreement is: if you know exactly who on points to (me and my friends, my dog, etc), then use the agreement with the meaning; else use the singular form.

Another rule is: don't use on if you're not sure. You can always use nous, il or ils instead.