Explanation: Just to elaborate a little on the answer, it can be added that the subjunctive mood refers to an action or a state that is not factual. The phrases "I believe that," "In my opinion," and "I think that" can be followed by the indicative mood in Spanish, but the phrase "I doubt that" must be followed by the subjunctive. You can say "Yo dudo que el viernes <em>llueva</em>," for instance, but you cannot say "Yo pienso que el viernes <em>llueva</em>."
