In Spanish, as in English, besides considering the concept of subject-verb agreement, a writer must consider the concept of grammatical mood, i.e., the attitude towards the information being presented, to select the adequate tense, i.e., the moment in time that an action, process, or state of being occurs.
The three different moods a writer can assume towards the information presented are the indicative, the subjunctive, and the imperative.
RAE’s Diccionario de la lengua española indicates that the indicative mood is used “to mark what is expressed by the predicate as information that is real”. This mood is used in the present, imperfect, preterit, future, and conditional simple tenses as well as in the present perfect, pluperfect, past anterior, future perfect, and conditional perfect compound tenses, in the first person, second person, and third person.
RAE’s Diccionario de la lengua española indicates that the subjunctive mood is used “to mark what is expressed by the predicate as information that is virtual, unspecified, unverified, or not experienced”. This mood is used in the present, imperfect, and future simple tenses as well as in the present perfect, pluperfect, and future perfect compound tenses in the first person, second person, and third person.
RAE’s Diccionario de la lengua española indicates that the imperative mood “mainly denotes orders, pleas, or appeals”. This mood is used in the present tense, mostly in the singular and plural second person, but there are a couple of exceptions wherein the first person and the second person are used.
Although independent clauses are usually written in the indicative mood, they may also be written in the subjunctive and imperative moods; subordinate clauses are written either in the indicative or subjunctive moods.
According to section 25.2 of Nueva Gramática, modal information is either dependent or independent. Dependent modal information is brought about by inductors, which are words, such as verbs, adverbs, nouns, or prepositions, as well as punctuation, such as question marks, and negation forms (no) that provide the attitude, i.e. mood, of the writer towards what is being presented.
Word: Quiero que escribas un libro.
Question mark: ¿Ves un libro que te interese?
Negation: Ella no cree que él venga.
Note that words and punctuation marks may induce the subjunctive mood; some of the sentences with these inductors may maintain the indicative mood.
Independent modal information does not need these inductors, which can appear either in an independent clause or a subordinate clause, to induce the mood of the subordinate clause (1870-1875).