"Jestem pewien" and "jestem pewny" is the same thing, it means that someone who is a male is 100% sure or steady and calm, ready for action. I, Arthur, am sure that I'm a man - Ja, Artur, jestem pewien/pewny, że jestem mężczyzną. "Jestem pewna" is a feminine form. I, Małgorzata, am sure that I'm a woman - Ja, Małgorzata, jestem pewna, że jestem kobietą. Artur has a steady hand in this match - Arur ma [pewną rękę] (because ręka is a feminine) w tym starciu. "Jest pewne" is a neuter form Dziecko jest pewne - A kid is sure In plural: "Oni są pewni (masculine form) - if there are only males in a group or the group is mixed "One są pewne (feminine f.)" - if there are females only in a group or the group consist of "neutral individuals" (like children, animals) You can also say "pewien pan, pewien człowiek, pewna kobieta, pewna książka, pewne dziecko" etc. For example - Pewien mężczyzna/pan zjadł tego pączka - There was a man who ate the pączek/Someone who was a male ate the pączek. In those cases "pewien/pewna/pewne" means that... I'm not sure what exactly this is. I suppose that both cases ( 1) jestem pewny - I'm 100% sure and 2) pewien pan - Someone who was a man but I'm not sure who he really was/pewna książka - some kind of book/ a book) can confuse you at the beginning :P "Pewny" might have also a bit different "use" like, in example: - "pewny towar" - a guy who tries to sell something want to assure buyer that the thing is not stolen or that the thing is the best quality (beer, drugs etc). Nic kradzionego, pewny towar! - "pewny grunt" - solid ground - in both methaphorical and purely physical meaning. Rozmowa zeszła na tory, które są dla mnie pewnym gruntem. Dom stoi na pewnym gruncie.

Well, ain't that something. Thanks!

No, "Jestem pewien" is only masculine.

But does Jestem pewna = Jestem pewien? I'm not confused about pewny/pewna, I'm confused about pewien. As a female, can I say Jestem pewien?

https://pl.wiktionary.org/wiki/pewny You can use both: Jestem pewny. = Jestem pewien. but "pewien" can be used only in nominative case . pewna - feminine gender