El subjuntivo es una forma verbal que les cuesta mucho entender a los extranjeros.The subjunctive is a verbal form that foreigners find very difficult to understand.
An adjective is a word that describes a noun (e.g. the big dog).
Recién cuando empecé a estudiar idiomas extranjeros me enteré de la existencia del modo subjuntivo.I only learned of the existence of the subjunctive mood when I began to study foreign languages.