Indicative vs Subjunctive in Relative Clauses

A relative clause or adjective clause is one that functions as an adjective.

Relative clauses describe or give extra information about a noun. In the above examples the clauses are giving us that information about “el libro” or “un libro”. In the example: “Busco el libro que costaba menos.”, the indicative is used in the clause that describes an identified noun. In the example: “Busco un libro que sea económico.”, the subjunctive is used in the clause that describes an unidentified noun. View the examples in the following image. Touch the upper-right-hand corner to enlarge.

This is closely related to how we use the indicative in noun clauses to assert an idea. In relative clauses we use the indicative to assert that we have identified the noun we are describing. Likewise, it is similar to how we comment on a virtual idea by using the subjunctive in a noun clause. In relative clauses we use the subjunctive when we can’t assert that we have identified the noun we are describing. This happens when we describe a noun that is hypothetical, unidentified, or one that doesn’t exist.

When we describe the characteristics of how something is done, where something is done or where something is, we use the indicative when we have identified how or where, and the subjunctive when we haven’t identified how or where.