13. Y, to it, to them, thereto, of it, is invariable. It is used of things. Adverbially it means there.

J'y pense nuit et jour,
Je vous promets d'y être,
 I think of it day and night.
I promise you to be there.

§ 30. Indefinite Pronouns.

1. These pronouns refer to persons and things in a general way:

autrui, others.
chacun, every one.
quelqu'un, some one, somebody.
quiconque, whoever.
l'un l'autre, one another.
 on, one, people, they.
personne, no one, nobody.
l'un et l'autre, both.
tel, such, many a one.
tout, everything, all.

2. A autrui, to others, d'autrui, of others, apply only to persons.

Ne fais à autrui ce que tu ne voudrais pas qu'on te fit,
C'est de la folie de s'affliger des succès d'autrui or des autres,
 Do not unto others that which thou wouldst not like to be done unto thee.
It is folly to grieve at the success of other people.

3. Chacun, chacune.

Chacun pour soi et Dieu pour tous,
Les langues ont chacune leurs bizarreries,
 Everybody for himself, and God for us all.
Each language has its oddities.

4. On conveys the idea of plurality, and is always the adjunct of a verb in the third person. See the auxiliary verb avoir, § 31.