Hello Lolita! 'I don't believe' is correct. 'don't' being a contraction of the words 'do' and 'not' For example: I do believe. I do not believe. finally , I don't believe.

Correct sentence is 2

Ana - the best way to understand an English sentence it to avoid 'don't', (also isn't, it's) try saying "do not" then sometimes it makes more sense. Really you should avoid using so at the end of a sentence. Some people would say " I do not believe it is so" but I " Do not believe it" is more fluent and easily understood. Of course this could only be in a sentence where you know what "it" is being referred to in the sentence.

"I don't believe someone/something.", is correct, as all other friends mentioned. On the other hand, there is such a phrasing as, "I am not believed by someone/some people.", when we are using "passive voice".

I don't believe.