A clause is a group of words that expresses an idea and contains a subject and a conjugated or “finite” verb (in contrast to an “infinite” or non-conjugated form such as the infinitive). A sentence will have one or more main clauses, and may have one or more dependent clauses or none at all.

• Espero que vengas a la fiesta.
  I hope (that) you'll come to the party.

For purposes of this section on the subjunctive, noun clauses are dependent clauses which serve as the direct object or predicate complement of another verb (or as the subject of a verb), just as a noun can do. Please note that English frequently employs an infinitive in these cases, whereas Spanish frequently requires a conjugated verb.

• Quiero el libro.
  I want the book.

El libro/the book is the direct object.

• Quiero que compres el libro.
  I want you to buy the book.

In English the direct object is the phrase "you to buy the book". The literal equivalent of the Spanish sentence is: "I want that you buy the book", and the clause "que compres el libro" is the direct object of the verb Quiero.

In the above example involving a dependent clause —“I want that you buy the book”— please note that:

• The governing verb (the verb which governs the dependent clause) is “want / querer” and that it expresses influence.
• The subject of the governing verb is “I / yo”.
• The subject of the dependent clause is “you / tú”, different from the subject of the main verb (“I / yo”).
• The verb in the dependent noun clause is “buy / compres”; however, the clause does not express a fact such as “you are buying the book” but rather that it is my desire “that you might buy the book”.

The rule: In Spanish, the subjunctive mood is used for the verb in a dependent noun clause when:

1. The subject of the governing verb is different from the subject of the dependent clause [e.g., “you / tú” vs. “I / yo” in the above example],

and

2. The governing verb is one of:

• Influence or willing [want, prefer, desire, insist, request, etc.], or
• Emotion [fear, be angry, be sad, be happy, be surprised, etc.], or
• Doubt or negation [be uncertain, be unsure, doubt, deny, etc.], or is an
• Impersonal expression of influence, emotion, doubt, probability, possibility, necessity, or a subjective reaction on the part of the speaker [e.g.,: It's urgent / bad, wonderful / uncertain / possible / probable / unlikely, etc.]

In contrast: The infinitive is normally used when there is no change in subject (I want to leave = Quiero salir), and the indicative mood is used when the governing verb expresses knowledge (to know) certainty (to be certain / sure), truth (to be true / the truth), affirmation (to believe, think, affirm, assert, declare), or reporting (to say, indicate [when not used as a verb of influence], report).