Quick answer
el libro que leí · la mujer con quien hablé
que covers almost everything. English lets you drop the relative word; Spanish never does.
que is the default
It works for people and things, as subject and as object.
English drops "that" in the first and third sentences. Spanish cannot.
After a preposition
| Situation | Use |
|---|---|
| no preposition | que |
| preposition + person | quien / quienes, or el que |
| preposition + thing | el que / la que / el cual |
Spanish never strands the preposition at the end of the clause the way English does.
Whose, and the neuter
cuyo means "whose" and agrees with the thing owned. lo que refers back to a whole idea.
Memory trick
If you can delete "that" or "who" in the English sentence, you still need que in Spanish. And the preposition always moves to the front.
Typical mistakes
El hombre hablé con es médico.
El hombre con quien hablé es médico.
Spanish cannot leave the preposition at the end or drop the relative pronoun.
La película que vi ayer era larga. / La película vi ayer era larga.
La película que vi ayer era larga.
Que is obligatory even when English omits "that".
El hombre que su coche es rojo.
El hombre cuyo coche es rojo.
Whose is cuyo, and it agrees with coche, not with the man.
Check yourself
Es la amiga con ___ viajé a Perú.