Spanish has a whole separate set of verb endings -- the subjunctive -- reserved for sentences that aren't simply stating a fact. It shows up after wishes, doubts, emotions, recommendations, and impersonal judgments, almost always triggered by a specific word or phrase earlier in the sentence. Learn the triggers and the mood mostly takes care of itself.
Indicative states what's real. Subjunctive shows up the moment a sentence expresses a wish, a doubt, an emotion, or an attempt to influence someone else.
Stating what's real or certain
Triggered by wishes, doubts, emotions, and influence
Creo que tiene razón, pero no creo que sea fácil.
I think she's right, but I don't think it'll be easy.
Creer que triggers the indicative when it states a belief, but no creer que flips to subjunctive -- denying certainty is itself a kind of doubt.
Take the quiz and see if you can spot the triggers that call for the subjunctive.