Pensar (subjuntivo)
I understand that a common trigger for the use of the Subjunctive in Spanish, is the prescence of 2 different subjects in a sentence.

Does this exclude certain verbs? For example, the use of Creer or Pensar?

Por ejemplo:
Pienso que debas comprarla o Pienso que debes comprarla.
I think you should buy it (but I don't know if you will, therefore I use the Subjuctive mood).

Is it wrong to assume that if there are 2 different subjects, then the Subjunctive applies to ANY verb?

pd. Perdóneme por escribir en inglés, pero es más facil para explicarme.

Question

Pensar (subjuntivo)
I understand that a common trigger for the use of the Subjunctive in Spanish, is the prescence of 2 different subjects in a sentence.

Does this exclude certain verbs?  For example, the use of Creer or Pensar?

Por ejemplo:
Pienso que debas comprarla o Pienso que debes comprarla.
I think you should buy it (but I don't know if you will, therefore I use the Subjuctive mood).

Is it wrong to assume that if there are 2 different subjects, then the Subjunctive applies to ANY verb?

pd.  Perdóneme por escribir en inglés, pero es más facil para explicarme.

Answer

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….”

Answer

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.

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