Indicative: Used when the antecedent (the person or thing) is known, real, or definite in the speaker’s mind.

Subjunctive: Used when the antecedent is unknown, hypothetical, non-existent, or when you are searching for someone/something that may not exist.

Example: “Tengo un amigo que habla (indicative/known) francés. Busco un amigo que hable (subjunctive/unknown) francés.”