penser que

Example

Je pense qu'il a raison. / Je ne pense pas qu'il ait raison.

Do not write

Je pense qu'il ait raison.

Why

A CLASSIC FLIP. In the affirmative you present the fact as true: indicative. In the negative and the interrogative you cast doubt on it, and the subjunctive becomes the norm. The indicative stays possible in the negative when the fact is still held to be real.