Demonstrative pronouns

Demonstrative pronouns have the following functions.

1. They replace a noun already mentioned. 

C’est ton livre ?

Ce n’est pas mon livre. C’est celui de Priya. 

2. Celui-ci and Ceux-là are used when a choice is indicated.
    this one        that one 

J’aime ces gâteaux.

Tu veux celui-ci ou celui-là ?
 

3 They come along with a relative pronoun.

They replace a noun already mentioned and functions as subject of the verb:

Celui qui, celle qui

the one that, the one who, and
 

ceux qui, celles qui 

those that, who 

Qui est Meena ?

C’est celle qui t’a parlé.

Who is Meena?  She is the one that talked to you.
 

Celle qui replaces the noun already mentioned Meena and functions as subject.

 

As object of the verb: 

Celui que, celle que

the one that / whom / which, 

ceux que, celles que

the ones that, whom, which 

Qui est cet homme ? C’est celui que j’aime.

Who is this man?    This is the one I love.
 

Celui que replaces he noun already mentioned and functions as object.

 
As object of the proposition ‘de’


Celui dont, celle dont

the one whose, the one of which, 

ceux dont, celles dont 

those whose, the ones of which 

De qui tu parles ? 

Tu ne connais pas celui dont je parle ? 

Whom are you talking about?

You don't know the one I am talking about!  

Celui dont is the object of the proposition ‘de’

When followed by de, demonstrative pronouns can indicate possession.

 
Ce n'est pas ma raquette, c'est celle de mon père.

It is not my racket, it is my father's. 

Demonstrative pronouns followed by a proposition and a relative pronoun.
 
Ce stylo est celui avec lequel le professeur a corrigé le devoir.

Ceci, cela and ça : 

These are called invariable demonstrative pronouns.

These are used to refer to an entire clause or to a vaguely described object. They do not refer to specific words, which have a known gender and number.

 
Cela and ça refer to something that has already been mentioned.

Ceci refers to something that is about to be said.
 

Mon fils ne m’écoute plus. Cela m'inquiète beaucoup.

Je vous dirai ceci : rien dans ce livre n’est tant difficile.