Normally "ne pas penser" is used with the subjunctive because it expresses doubt:
Je ne pense pas qu'il aille à la fête ce soir.
If I'm sure of the thing that I'm saying (in this case, I'm sure he's not going to the party), can I use the indicative? Does the answer differ for written and spoken language?
Je ne pense pas qu'il va aller à la fête ce soir.