grammar.pronouns.relative-que
The relative que: the all-purpose 'that/which/who'
Que links a clause to any noun, person or thing, and is never omitted: el libro que leí, la persona que llamó.
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Explanation
Que is the workhorse relative pronoun — 'that / which / who' — linking a clause to almost any antecedent, person or thing, singular or plural, with no change of form: el libro que leí, la persona que llamó, las casas que vimos.
Unlike English, Spanish never omits it. 'The movie I saw' must be la película que vi, not *la película vi; 'the people we met' is la gente que conocimos. Dropping que is one of the most common English-speaker errors.
After short, common prepositions, que takes an article: la empresa en la que trabajo, el tema del que hablamos. For people, after a preposition, quien is also available (la persona con quien / con la que hablé). In non-restrictive clauses (set off by commas), que still works: mi vecino, que es médico, …
Examples
The apartment (that) we visited is for sale.
Region: global
The lady who lives upstairs is a doctor.
Region: global
The company that hired me is the same one you work for.