In short

Lesson 38 gave you the everyday relatives - que (the all-purpose one), quem (people, after a preposition) and onde (places). This lesson adds the two heavier relatives that careful, written European Portuguese leans on. O qual - with its forms a qual, os quais, as quais - is a longer, more explicit stand-in for que and quem: you reach for it after a preposition, to break an ambiguity, or inside a comma (non-restrictive) clause. Cujo - cuja, cujos, cujas - is the single word for “whose”, and it hides one sharp trap: it agrees not with the owner but with the thing owned, takes no article, and clamps straight onto the noun. Neither is optional polish: after a compound preposition EP refuses bare que, and cujo is the only correct way to relativize possession.

The rule

O qual has four forms and agrees with its antecedent

O qual is really the article o/a/os/as welded to qual, so it carries the gender and number of the noun it points back to (its antecedent):

O senhor a quem escrevi respondeu uses quem; the more formal o senhor ao qual escrevi respondeu uses o qual, and qual becomes a qual, os quais, as quais to match whatever it refers to.

When to use o qual instead of que

Bare que is still the default for a simple subject or object relative (o livro que comprei). Switch to o qual in three situations:

1. After a preposition - and obligatorily after a compound preposition. With the simple prepositions de, em, por, a, com + a thing, both que and o qual occur, but after multi-word prepositions (ao lado de, por causa de, graças a, através de, dentro de, a respeito de) EP requires the qual/quem form and never bare que:
   • o motivo pelo qual vim - the reason I came.
   • o prédio ao lado do qual moro - the building next to which I live. (never ao lado de que)
2. To remove ambiguity. Because o qual agrees, it can point cleanly at one of two possible antecedents where que would be vague: A irmã do médico, a qual vive em Lisboa, … (a qual, feminine = the sister) versus o qual, … (= the doctor).
3. In non-restrictive (comma) clauses in careful or written register: O relatório, o qual foi divulgado ontem, provocou reações.

The preposition fuses with o qual, exactly like with an article

Every contraction you learned in Lesson 2 (and por -> pelo from Lesson 28) applies here unchanged, because o qual begins with an article:

So it is na qual, pelo qual, à qual - never em a qual, por o qual, a a qual.

People after a preposition: quem or o qual, never que

For a person after a preposition, EP uses quem (Lesson 38) or, more formally, o qual - but not bare que:

• o amigo com quem falei = o amigo com o qual falei - the friend I spoke to.
• NOT o amigo com que falei (heard regionally, but outside the standard).

Cujo: the one word for “whose”

Cujo joins a possessor to a possessed thing and translates “whose” or “of which”. It has four forms and one iron rule: it agrees with the thing possessed, not the owner, carries no article, and sits immediately before that noun:

• o escritor cujos livros li - the writer whose books I read. (cujos agrees with livros, not with escritor.)
• a empresa cujo diretor conheço - the company whose director I know.
• um problema cuja solução ninguém encontra - a problem whose solution nobody can find.

A preposition can precede cujo; it still agrees with the possessed noun and never contracts, since there is no article to fuse with:

• a casa em cujo jardim brincávamos - the house in whose garden we used to play.
• a autora a cujo estilo me habituei - the author whose style I got used to.
• o tema sobre cujas implicações discutimos - the topic whose implications we discussed.

Forms & patterns

Choosing the relative after a preposition:

Cujo agrees rightward - look at the noun that follows it:

Examples

Common mistakes

• Que after a compound preposition: a razão por causa de que faltei -> por causa da qual - multi-word prepositions demand the qual form and its contraction.
• An article after cujo: o homem cujo o carro bateu -> cujo carro - cujo already contains the idea of the article; nothing may sit between it and the noun.
• Making cujo agree with the owner: a mulher cujo filhos vieram -> cujos filhos - it agrees with filhos (m. pl), never with mulher.
• Using de que / de quem for the “whose” of a thing: a casa de que as janelas estão partidas -> cujas janelas - possession of a thing is always cujo.
• Dropping the contraction: o fim por o qual lutamos / a sala em a qual entrámos -> pelo qual, na qual.
• Bare que + preposition for a person: o colega a que pedi ajuda -> a quem (or ao qual).
• Scattering o qual where plain que is natural: O bolo o qual fiz -> que fiz - without a preposition, in a restrictive clause, que is the correct default; o qual there sounds stilted.
• In Brazil: the written norm is identical - o qual and cujo work exactly the same way. What differs is only casual speech, where Brazilian Portuguese often avoids both with a resumptive pattern (o autor que eu li os livros dele); that is nonstandard on both sides of the Atlantic, so keep cujo in writing.

Tips

• After a preposition, ask two questions. Thing or person? Thing -> o/a qual (or bare que with the simple prepositions); person -> quem (or o qual). Simple or compound preposition? Compound -> always the qual/quem form, never que.
• Cujo looks forward, not back: it takes its ending from the noun that follows it and snaps straight onto it, with no o/a in between - cujo carro, cujas ideias.
• Contract on autopilot. Any preposition that fuses with o/a/os/as fuses with o qual too - do qual, no qual, pela qual, à qual - but cujo has no article, so it stays loose: a cujo, de cujo, em cujo.
• Keep o qual in reserve. Plain que remains the everyday relative; reach for o qual only to (a) follow a preposition, (b) break an ambiguity between two possible antecedents, or (c) open a comma clause in careful writing.