Learn Hardcore French: Le canapé sur lequel le chat dort est confortable. - The couch on which the cat sleeps is comfortable.

So sur lequel connects the noun le canapé to the relative clause le chat dort, while keeping the preposition sur attached to it (as French does not leave prepositions at the end of the clause like English can).

Why do we use lequel and not qui or que?

In French, the choice of relative pronoun depends on its role and whether there is a preposition:

qui is used as a subject:
- Le chat qui dort est mignon. = The cat who is sleeping is cute.
que is used as a direct object:
- Le canapé que j’ai acheté est confortable. = The sofa that I bought is comfortable.
lequel / laquelle / lesquels / lesquelles is used when the relative pronoun comes after a preposition (like sur, avec, dans, pour, sans, etc.):
- Le canapé sur lequel le chat dort est confortable.
  = The sofa on which the cat sleeps is comfortable.

Here, because of the preposition sur, we cannot use qui or que; we must use lequel.

Could we use où instead of sur lequel here?

Yes, you can say:

Le canapé où le chat dort est confortable.

Où can sometimes replace préposition + lequel when it refers to a place. A canapé is a location where the cat sleeps, so French often allows où.

However:

sur lequel is a bit more explicit and slightly more formal.
où is more common in everyday speech when talking about places.

Both are grammatically correct in this sentence.

How does lequel agree with canapé? What are the other forms of lequel?

Lequel must agree in gender and number with the noun it refers to.

le canapé is masculine singular → lequel

The full set is:

Masculine singular: lequel
Feminine singular: laquelle
Masculine plural: lesquels
Feminine plural: lesquelles

Examples:

La chaise sur laquelle le chat dort… (chair – feminine singular)
Les canapés sur lesquels le chat dort… (sofas – masculine plural)

Why can’t I say something like Le canapé que le chat dort sur est confortable, like in English?

French does not allow you to leave the preposition at the end of the clause. You must place the preposition directly before the relative pronoun.

So:

English can say: the sofa that the cat sleeps on
(preposition at the end)
French must say:
- le canapé sur lequel le chat dort
  or le canapé où le chat dort

✗ Le canapé que le chat dort sur is incorrect in French because sur cannot be stranded at the end.

Why is it le chat and not just chat like in English?

In French, common nouns almost always need an article (definite, indefinite, or partitive). You cannot normally use a bare noun the way English does.

le chat = the cat (a specific one, or the “known” cat)
un chat = a cat (non-specific)
les chats = the cats

English can say just “cats” or “cat” in some contexts (like general statements), but French needs something like le, la, les, un, une, des, du, de la.

So in this sentence, le chat is natural because we are talking about a specific cat that the speaker and listener presumably know about.

Why is the verb dort and not something like est en train de dormir?

Dort is the simple present tense of dormir (to sleep). In French, the simple present covers both:

an action happening right now
(The cat is sleeping.)
a habitual action
(The cat sleeps on this sofa.)

French usually uses the simple present rather than a progressive form. Être en train de + infinitive exists (e.g., est en train de dormir), but it is used only when you want to emphasize that the action is in progress right now.

In a general descriptive sentence like this, dort is the most natural choice.

What does confortable agree with, and why is it masculine singular?

Confortable is an adjective describing le canapé.

le canapé → masculine singular
So confortable stays in the masculine singular form.

Unlike some adjectives, confortable has the same spelling in masculine singular and feminine singular:

Masculine singular: un canapé confortable
Feminine singular: une chaise confortable
Masculine plural: des canapés confortables
Feminine plural: des chaises confortables

In the sentence, there is only one sofa, so confortable matches that: masculine singular.

Could the word order be Le chat dort sur le canapé, qui est confortable instead?

You can say:

Le chat dort sur le canapé, qui est confortable.

This is grammatically correct, but it changes the nuance:

Le canapé sur lequel le chat dort est confortable.
Focuses on the specific sofa defined by the fact that the cat sleeps on it. It’s like identifying which sofa you mean.
Le chat dort sur le canapé, qui est confortable.
First states a fact (the cat sleeps on the sofa), then adds extra information about the sofa (it is comfortable). The relative clause is more like an afterthought.

Both are acceptable, but the original is more tightly focused on identifying which sofa.

Is canapé always “sofa”? Are there other words I should know?

Canapé usually means sofa / couch.

Other related words:

un sofa – also sofa, but canapé is more common in everyday French.
un divan – a couch or daybed (a bit more specific; sometimes more old-fashioned or literary).
un fauteuil – an armchair, not a sofa.

In a typical modern context, canapé is the standard word for sofa.