I'm trying to understand better why the subjunctive is used. Are negative opinions like no creo que, no opino que, no pienso que, no parece que etc, always assumed to reflect an element of doubt on the part of the person i.e. "I don't think so .... but I may be wrong."
If you are adamant that the negative opinion is correct [for example using one of the examples in the associated lesson] "I don't think María is jealous," couldn't that also be taken as a clear statement of my opinion without any doubt in my head at least? This would be possible in English. In which case would it be expressed differently in Spanish for example "Estoy seguro de que María no es celosa."