"Maybe they are thinking that you are my husband."
Translation:Może myślą, że jesteś moim mężem.
It's slightly different. "Może myślą, że..." is kind of a suggestion, hence maybe (literal translation). "Chyba myślą, że..." can be translated as "They probably think that..." or "I think/guess they think that...". Using chyba you identify more with the opinion.
"moich" is the wrong form (plural), otherwise it's okay.
Although using 'oni' explicitly gives an unusual emphasis on "they", like "Maybe THEY think..." - if you are to use such a sentence, it seems obvious that your interlocutor knows from the context who 'they' are.
I may be a little to the conversation but can someone send me a good break down when to use which case and how cases alter words in singular and plural. I know polish is a very sensetive language and there probably are a lot of special cases where a general rule doesnt fit, but Im having big trouble to find out when to use which case in the first place. The specials and vocabulary may come with time.
Thank in advance!