How should I use the 'subjonctif'(subjunctive) with the 'verbes d'opinion'?

The 'subjuctif' is used to express willingness, obligation, a suggestion, a feeling, a judgement, a possibility, a doubt..

When we want to express an opinion using verbs such as espérer, croire, dire, affirmer, être certain/e, imaginer in our principal clause, it is not to be taken for granted that in the following clause we are going to use a verb in the subjuctive mood.

In fact, some exceptions apply:

if the principal clause is in the AFFIRMATIVE FORMS, it is followed by a verb in the INDICATIVE FORM.

e.g. Je crois qu'il est déjà arrivé a Nice.

if the principla clause is either in the NEGATIVE and the INTERROGATIVE FORM, it is followed by a verb in the SUBJUCTIVE MOOD.

e.g. Je ne crois pas qu'il soit arrivé a Nice.

Crois-tu qu'il soit arrivé a Nice?