Relative clauses are essentially clause-long modifiers like adjectives and prepositional phrases. What distinguishes them is that they are a whole clause long! Here are three different ways to say the same thing using those three different types of modifiers.
Adjective: I saw a three-legged dog. Prepositional Phrase: I saw a dog with three legs. Relative Clause: I saw a dog that had three legs.
The relative clause example in Latin would be:
Canem quī tria crura habē
The main clause is canem vī dī(“I saw a dog”). The relative clause tells what kind of dog I saw. As you know, for a clause to be a clause, it must have a subject and a predicate. For the main clause, that separation is obvious. For the relative clause, we'll have to look more closely.
The sense of our relative clause is that the dog had three legs. You could split the sentence in two:
Canem vī dī. (I saw a dog.) Canis tria crura habē bat. (The dog had three legs.)
Both sentences include the word canis. In the first sentence, canem is accusative case because it is the direct object of vī dī. In the second sentence, canis is nominative case because it is the subject of habē bat. Since it is the job of pronouns to stand in place of nouns, you can link the two sentences by replacing one of the dogs with a pronoun. If you use a relative pronoun, you can change one of the sentences into a modifier — a relative clause.
If you want to make the canem of the first sentence modify the canis in the second one, you need to replace it with a relative pronoun. But which one? Well, canem is masculine accusative singular, so you need a masculine accusative singular form of quī, which is quem. Now the two sentences are:
|quem vī dī||Canis tria crura habē bat|
Since relative clauses are modifiers and in Latin modifiers tend to follow the words they modify, insert the relative clause after the word it's modifying in the second sentence. The result is: Canis quem vī dī tria crura habē bat. (“The dog that I saw had three legs.”) That sentence tells which dog had three legs — the one I saw did.
If you substituted a relative pronoun for the dog in the other sentence, you'd follow the same procedure. Since canis is masculine nominative singular, you need the masculine nominative singular form of the relative pronoun, which is quī.
|Canem vī dī||quī tria crura habē bat|
Put them together and you get: Canem quī tria crura habē bat vī dī.(I saw a dog that had three legs.) This version tells what kind of dog I saw — one that had three legs.
The key points to keep in mind regarding relative clauses are:
A relative pronoun takes its gender and number from its antecedent (the word it modifies).
Arelative pronoun takes its case from its own function in its own clause.
Since relative pronouns stand in for nouns, they can have any case and any case use a noun can have.
Nominative: Canem quī tria crura habē bat vī dī. (I saw a dog that had three legs.)
Ablative: Oppidum in quī habitat parvum est. (The town in which he lives is dinky.)