Acho Que or Não Acho Que? Indicative vs Subjunctive

The mood of the second verb depends on what the first part of the sentence says about it.

Use the indicative for facts and for what you believe to be true: achar que, pensar que, saber que, ter a certeza de que, é verdade que.
Acho que ele tem razão. (I think he's right.)

Use the subjunctive for doubt, denial and wishes, and after certain conjunctions:

• não achar que, duvidar que, querer que, esperar que
• embora (although), para que (so that), antes que (before), caso (in case), sem que (without)

Não acho que ele tenha razão. (I don't think he's right.)

Watch the negation: acho que takes the indicative, but não acho que takes the subjunctive, because the speaker no longer presents the idea as true.