LES PRONOMS DEMONSTRATIFS

I The variable demonstrative pronouns are "celui," "celle," "ceux," "celles." They are variable because they agree in gender and number. They are used to translate into French "the one(s) (that), "the one(s) (which), "the one(s) (who)" (never use "l'un"). Variable demonstrative pronouns can be used with any relative pronoun.
Ex.

1. Among the pens, give me the one that works.=Parmi les stylos, donnez-moi celui qui marche.
2. Do you see the one (who) I spoke about?=Voyez-vous celui dont j'ai parlé?
3. Show me the one (that) you spoke to.=Montrez-moi celui à qui tu as parlé.

Some verbs that are followed by the prepositions "de" and "à" or by a direct object are:

Verbe + de +complément
avoir peur de
avoir besoin de
douter de
être amoureux de
se souvenir de
être content de
écrire de
Verbe + à
penser à qqn/qqch
réfléchir à qqch
assister à qqch
raconter qqch à qqn
rendre visite à qqn
obéir à qqn
parler à qqn
téléphoner à qqn
s'intéresser à qqn/qqch
Verbe + object direct
attendre qqch
demander qqch
dire qqch
écouter qqch
écrire qqch
expliquer qqch
promettre qqch
raconter qqch
suggérer qqch

II Variable Demonstrative Pronouns are also used with "de" to indicate people or things possessed by someone or something.
Ex.

1. I am thinking about your ideas and Mari-Claire's.=Je réfléchis à tes idées et à celles de Marie-Claire.
2. She is cleaning the apartments on the first floor and those on the second floor.=Elle nettoie les appartements du rez-de-chaussée et ceux du premier étage.

B. The French use "cela" and "ça" much less frequently than Americans use the equivalent pronouns, "this" and "that," especially in writing. The French prefer to replace "cela" and "ça" with the demonstrative adjectives "ce," "cet," "cette," or "ces" followed by a noun that resumes the entire idea.
Ex.
Il a pris le train?
That surprises me.=Cette action me surprend.