"I do not think so."
Translation:Eu acho que não.
Well if you think about it in English... "I do not think so" is technically a shorter way of saying "I do not think that it is so" or something along those lines. If you want to try (although it isn't always possible) to see this expression as a one-to-one correspondence of the English phrase, you could see it as "I do not think QUE it is so." Do you see what I mean?