Subjunctive with doubt and opinion

Verbs of doubt or negated belief are followed by the subjunctive: dudar que, no creo que, no pienso que, no estoy seguro de que.

Important contrast: creo que (certainty) → indicative, but no creo que (doubt) → subjunctive.

Impersonal expressions of possibility or probability require the subjunctive, because the outcome is not certain: es posible que, es probable que, puede que, es dudoso que.

Quizás and tal vez ('maybe') can be followed by either the subjunctive or the indicative:

• Subjunctive → more doubt, less certain
• Indicative → more conviction, almost certain

In practice, most speakers choose the subjunctive, especially when quizás/tal vez comes before the verb.

Expressions that convey certainty use the indicative, not the subjunctive: es verdad que, es cierto que, es obvio que, estoy seguro/a de que.

Note: when you negate these expressions (e.g. no es verdad que), it becomes subjunctive again, because you're then expressing doubt or denial.