Think so

The Phrases I think so and I don’t think so are nearly always used on their own as a complete sentence. They are used as a way of simply saying yes or no to answer a yes/no question.

Are you going to the party? – Czy idziesz na imprezę?
I think so. – Myślę, że tak.

Are you going to the party? – Czy idziesz na imprezę?
I don’t think so. – Myślę, że nie.

Instead of using I don’t think so, you can also use I think not. However, this isn’t used as often because it is much stronger and can suggest that this is something you would never do.

Can I stay out till midnight? – Czy mogę wrócić o północy?
I think not! – Nie ma mowy!

Are you going to Piotrek’s party? – Czy idziesz na imprezę do Piotrka?
I think not! – Nigdy w życiu!

You never use I think so or I don’t think so when you give an opinion about something.

I think (that) this is their house. – To chyba ich dom.

I think so (that) this is their house.This is wrong!