Pienso que .................
Pienso que la reunión haya comenzado en la oficina.
I think that the meeting has started in the office.
2 Answers
In declarative sentences, "pienso que" will always take the indicative. When stating things that you believe to be true, the indicative is used.
Thus: Pienso que la reunión ha comenzado en la oficina.
Pienso que ....can also use the indicative to inform us of your opinion. (The meeting has probably started in the office (since it was scheduled to begin a half hour ago)
If you wish to express doubt or uncertainty (Pienso que, pero no estoy seguro.. the meeting was scheduled to start a minute ago, then the subjunctive is used). In many cases it expresses that you doubt that something happened. I doubt that the meeting has started yet. (contrary-to-fact or hypothetical)