In Spanish, we are going to use the subjunctive after verbs expressing doubts or uncertainty (check present subjunctive conjugation), and verbs saying what you think about something that is used with “no”. Here you have some examples:
No creo que sea importante. (I don’t believe it’s important.)
No pienso que esté bien. (I don’t think it is right.)
Dudo que tenga tiempo. (I doubt I have time.)
No pienso que entiendan. (I don´t think they understand.)
No estoy seguro que Olga venga. (I am not sure that Olga is coming.)
But we would use the indicative when the sentence is affirmative:
Creo que es importante. (I believe it’s important.)
Pienso que está bien. (I think it is right.)
No dudo que tendré/tenga tiempo. (I don’t doubt I have time.)
Pienso que entienden. (I think they understand.)
Estoy seguro que Olga vendrá. (I am sure that Olga is coming.)