Yes and no are probably the first words you learn in English. Are you going to the party? Yes. Are you going to the party? No.
But as your English develops a little, your sentence constructions expand to more than just one word answers. But whereas in Czech, you might answer the question, „Are you going to the party?“ with „I think yes“, in English it is wrong.
Instead you need to say „I think so.“ To say the opposite – if you don’t think that you will go, you have to say, „I don’t think so.“
Saying, „I think yes“ or „I think no“ is Czenglish, and you should cut it out right away!