Subjunctive with Adjective Clauses

Subjuntivo con cláusulas adjetivas

Adjective clauses are relative clauses: a relative pronoun (usually que) + some description that modifies a noun. Some adjective clauses require the indicative, while others need the subjunctive. What’s the difference? Reality and existence of the noun described by the adjective clause.

Verbs like buscar, deber, necesitar, and querer commonly require the subjunctive. Whether or not to use it depends on the speaker’s perception of reality.

Compare:

Busco el libro que estoy leyendo.   I’m looking for the book I’m reading. (I know it exists = no doubt)
Busco un libro que me interese.   I’m looking for a book that interests me.
(it may not exist = doubt)
    
Quiero el perro que tiene ojos negros.   I want the dog that has black eyes. (It’s sitting right here.
Quiero un perro que esté domesticado.   I want a dog that is housebroken. (I haven’t found it yet.)

When verbs are used in questions or negative statements, they may also followed by the subjunctive.

Conozco a alguien que habla italiano.   I know someone who speaks Italian (this person exists).
¿Conoces a alquien que hable italiano?   Do you know anyone who speaks Italian? (don’t know if this person exists)
No conozco a nadie que hable italiano.   I don’t know anyone who speaks Italian. (this person doesn’t exist)
    
Hay un libro que es interesante.   There’s a book that’s interesting.
¿Hay un libro que sea interesante?   Is there a book that is interesting?
No hay ningún libro que sea interesante.   There is no book that is interesting.
    
Tengo un profesor que vive en Madrid.   I have a teacher who lives in Madrid.
¿Tienes un profesor que viva en Madrid?   Do you have a teacher that lives in Madrid?
No tengo un profesor que viva en Madrid.   I don’t have a teacher that lives in Madrid.

There are some clues to help you decide between the indicative and subjunctive:

More Spanish Subjunctive

Conjugations

Usage

Spanish Subjunctive Tenses