EL SUBJUNTIVO EN CLÁUSULAS SUSTANTIVAS

The subjunctive is used in a noun clause when the main verb expresses
influence, emotion, denial or doubt.

•  INFLUENCE: After verbs expressing want, desire, hope, request, insistence, command, suggestion, etc., what is in the clause is like a command and must be in the subjunctive:
Espero que vayas. (I expect you to go).

•  EMOTION: When the main verb expresses any subjective attitudes (sorrow, surprise, anger, indifference, concern, hope), the verb in the noun clause must be in the subjunctive:
Me gusta que vayas (I like teh fact that you're going).

Verbs of influence and emotion precede the infinitive, not the subjunctive, when there is not another subject involved: Quiero que vayas (I want you to go [two subjects, subjunctive])
Quiero ir (I want to go [one subject, infinitive]).

•  DOUBT: When the main verb expresses doubt, disbelief or denial, the verb in the noun clause should be in the subjunctive: Dudo que vaya (I doubt that I'm going).

In general, creer, no dudar, ser verdad, and other expressions of conviction imply certainty and therefore will not require a subjunctive in the clause that follows, while no creer, dudar, no ser cierto, etc., will: Creo que irás.  (I think you'll go). No creo que vayas.  (I don't think you'll go).

•  Ojalá (an Arabic term used to express wish or hope) is always followed by the subjunctive.