Breakdown of Le canapé sur lequel tu finis ton livre est confortable.
être
to be
tu
you (informal singular)
sur
on
le livre
the book
Questions & Answers about Le canapé sur lequel tu finis ton livre est confortable.
Why do we use lequel here instead of que?
We need a relative pronoun that follows the preposition sur (on). We use que for direct objects (like in 'the sofa that I like'), but after a preposition referring to a thing, we must use a form of lequel.
Why is it exactly lequel and not laquelle or lesquels?
The relative pronoun has to match the gender and number of the noun it refers back to. Here, it stands in for le canapé (the sofa), which is masculine and singular. Therefore, we use the masculine singular form lequel. If it were a feminine noun like la chaise (the chair), we would say sur laquelle.
In English, I might just say 'The sofa you finish your book on'. Can I put sur at the end of the sentence in French?
No, French grammar never allows dangling prepositions at the end of a clause. The preposition must always come right before the relative pronoun it belongs to, forming sur lequel (literally 'on which'). Additionally, you can never drop the relative pronoun in French the way you often can in English.
I see tu finis; is that an irregular verb?
No, finir is a standard regular -ir verb. To conjugate it in the present tense for tu, you take off the -ir ending and add -is, giving you tu finis.