forms and uses
A relative pronoun introduces a clause that explains or describes a previously mentioned noun, which is called the antecedent. Relative pronouns are used to link two related ideas into a single sentence, thereby avoiding repetition.

In the above example, the relative pronoun qui introduces the subordinate clause, that is, the clause that adds additional information about the novel. In French, there are two main relative pronouns, qui and que. The choice between qui and que in French depends solely on the grammatical role, subject or direct object, that the relative pronoun plays in the subordinate clause.

• qui

Qui functions as the subject of the subordinate clause.

• que

Que functions as the direct object of the subordinate clause. Remember that que becomes qu’ before a word beginning with a vowel.

agreement
Although qui and que are invariable, they assume the gender and number of the antecedent. Que functions as a direct object preceding the verb. Therefore, when the verb of the subordinate clause is in the passé composé, or any other compound tense, the past participle agrees in number and gender with que. The past participle also agrees in number and gender with qui if the verb forms its passé composé with ‘être’.

A relative pronoun introduces a clause that explains or describes a previously mentioned noun. In instances where the relative pronoun is the object of a preposition, relative pronouns other than qui and que must be used. De is the most common of these prepositions, and dont is the relative pronoun representing both the preposition de + its object.

• dont
Use dont if the subordinate clause needs an object introduced by de/d’. Such clauses may indicate possession or they may contain verbs which are followed by the preposition de. Some of these verbs include ‘parler de’ (to talk about); ‘avoir besoin de’ (to need); ‘avoir peur de’ (to be afraid of); ‘tenir de’ (to take after).

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Dont often indicates possession; ‘whose’ is its English equivalent.

• où
The relative pronoun où means ‘where, in which, on which.’ Use où if the subordinate clause needs an object indicating location introduced by dans, à, sur, sous. When used after adverbs of time, où means ‘when.’

interrogative pronoun lequel – review

Lequel, lesquels, laquelle, lesquelles are pronouns, i.e. they are used in place of a noun. They are used to ask the questions ‘which one?’ or ‘which ones?’ They assume the number and gender of the nouns they replace and contract with the prepositions à and de.

relative pronoun lequel

• preposition + lequel, laquelle, lesquels, lesquelles

The relative pronouns lequel, laquelle, lesquels, and lesquelles (which) are used when the relative clause is introduced by a preposition other than de/d’, such as à, de as part of a 2- or 3-word prepositional phrase (à côté de, près de, etc.), avec, par, pour, sur, dans, etc.  These pronouns make the usual contractions with the prepositions à and de. Note that the preposition in French must always be placed immediately in front of the relative pronoun.

• contractions – review

The prepositions à or de contract with the definite articles le and les to give the following forms:

de + le = du.          de + les = des

à + le = au              à + les = aux

La and l’ on the other hand do not contract after à and de. Note that these contractions also occur with two- and three-word prepositions ending with à or de (jusqu’à, près de, loin de, à côté de, etc.).

The following information and tables below are from Français 202 (page 84) by William J. Carrasco & Shahrzad Zahedi is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) license.

General summary of relative pronouns:

Table 1:

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Table 2:

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The form dont is generally used in spoken French instead of the forms duquel, de laquelle, desquels, and desquelles; however, these latter forms may also be found, especially in written texts. Dont may be substituted only for the simple preposition de and its object, but a form of lequel must be used when de is part of a two- or three-word preposition, such as ‘à propos de, près de, loin de, à côté de.’

When you can ’t make a contraction, you place the preposition before the relative pronoun as follows :
préposition + qui, quoi, où, lequel, laquelle, lesquels, lesquelles

Exemples: préposition + lequel:

préposition + qui

Use qui instead of a form of lequel when referring to people.

préposition + où

NOTE: You will never add a préposition to dont because it already has the preposition de built into it.

The following information and table are from Français Interactif.

The form dont is generally used in spoken French instead of the forms duquel, de laquelle, desquels, and desquelles; however, these latter forms may also be found, especially in written texts. Dont may be substituted only for the simple preposition de and its object, but a form of lequel must be used when de is part of a two- or three-word preposition, such as ‘à propos de, près de, loin de, à côté de.’