That would be true in Italian, but not exactly in Spanish. In Spanish, we use the indicative for when you believe something, and the subjunctive when you doubt / don’t believe: 1. indicativo: Creo que debes…. 2. subjuntivo: No creo que debas…. 3. indicativo: Creo que no debes…. Notes: When talking about believe, rather than thought, it’s a bit more common to use “creer” than “pensar.” In English, it’s more common to say “I don’t think…” than “I think… not…,” but in Spanish, it’s more common to say “Creo que no…” rather than “No creo que….”

We use the Subjuntive in Spanish to express wishes, hope, feelings, probability, to give some advice or your opinion like a part of a subordinate clause that is usually linked by "que". For example, Tengo muchas ganas de que vengas a verme. Espero que hayas tenido buen viaje. Ojalá no llueva mañana Puede que me pase mañana por tu casa Deseo que tengas un buen día. Es probable que mañana nieve Tengo el frigorífico vacío, es necesario que vaya a comprar. When we use "creer" or "pensar" in negative form. No creo que sea buena idea No pienso que sea cierto If you use these verbs in the afirmative form, we use the indicative or the conditional if you want to give some advise or your opinion: Creo que tienes razón (opinion) Creo que deberías hacerme caso (advise) P.s. I'm improving my English so sorry if I have made many mistakes, but I wanted you to understand the different uses.