I think yes.
I think no.
I think I won't pass the exam.
All of these are clear enough--but not natural English. In the case of the first two examples, native speakers are more likely to say:
I think so.
I don't think so.
Note that we say this only in isolation, that is, if we're only going to make a short response to a question, or a fact or opinion that was already stated. For example:
--Is it getting colder outside? --I think so.
--I bet he's got a lot of money. --I don't think so.
We do not use this form when we're introducing an opinion or a fact. We don't say:
In that case, we'd just take out the word so and say:
I think that it's getting colder outside.
When we are expressing a negative opinion, that is, saying that we disagree, we are more likely to use this form:
I don't think I'm going to pass the exam.
In other words, we move the negative (not) so that it applies to the verb think.