Learn Polish: Chyba tak, ale nie wiem. - Probably yes, but I don't know.

The word chyba is used to express uncertainty, similar to "probably", "I guess", or "I think". The phrase chyba tak is the most natural way in Polish to say "I think so" or "probably yes".

Can I use chyba nie to mean the opposite?

Yes! Just as chyba tak means "I think so", you can use chyba nie to mean "I don't think so" or "probably not". They are both extremely common phrases in daily conversation.

Why is it wiem and not znam for "I know"?

Polish has two different verbs for "to know". You use wiedzieć (which becomes wiem for "I know") when talking about facts, information, or situations. You use znać when talking about being familiar with a person, place, or a specific thing. Since you are unsure about a fact here, wiem is the correct choice.

Where is the word for "I" in nie wiem?

In Polish, the subject pronoun ja (I) is usually dropped because the verb ending already shows who is doing the action. The -em ending on wiem clearly means "I know", so adding ja is unnecessary unless you want to add strong emphasis.