Demonstrative pronoms:
Celui Replaces a masculine singular noun.
Celle Replaces a feminine singular noun.
Ceux Replaces a masculine plural noun.
Celles Replaces a feminine plural noun.
Usage:
1. Demonstrative pronouns can be followed by –ci and –là.
Ci refers to anything referred recently / this and là refers to anything referred earlier / that.
Cette fille-ci est plus jeune que celle-là.
This girl is younger than that one.
Ce livre-ci est en anglais et celui-là est en français.
Ceux-ci sont meilleurs que ceux-là.
These are better than those.
2. Demonstrative pronouns can be followed by a relative pronoun.
Cette fille est celle que tu avais tant aimée.
This girl is the one whom you so loved.
Celui qui mange bien travaille bien.
He who eats well works well.
3. 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 fairly vaguely described object in the real world. They do not refer to specific words which have a known gender and number.
Cela and ça refer backward, to something that has already been mentioned.
Ceci, on the other hand, refers forward, 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.