Just an observation that I hope may help someone, as there are a couple of questions on here that express some confusion about this: it might make more sense at least in this case to think of the use of the subjunctive and indicative as marking verbs describing circumstances that are real vs those that are unreal. When you say "no opino que mi jefe tenga razón", the clause "...my boss is right" is decribing a situation that just doesn't exist, and actually the thesis of the sentence is precisely the opposite of that! So using the indicative would sound weird, because in effect your subordinate clause would sound like a factual statement contradicting the idea you're expressing. To avoid this, Spanish grammar uses the subjunctive in this context. In contrast, saying "opino que el jefe sí tiene razón" works fine because the part saying that "...the boss is right" alligns with the perceived fact you're stating. Of course it's not 100% logical all the time because every language is going to have its little quirks, but I think this is a more useful way to view it than the certainty-versus-uncertainty idea. Of course, there are many times when the subjunctive implies uncertainty--but that's more because uncertainty falls under a broader umbrella of unreality, not because the purpose of the subjunctive is to express uncertainty per se.
Anyone please feel free to correct or elaborate more on this if you wish. I hope someone else finds this helpful.
