하지만 is mostly used in formal writing. Saying the words used in formal writing sounds *pointed*, so I'd like you not to use this word when you're talking with people. Except when you're arguing with someone(you're gonna need to use cold language ^^;) 근데 is commonly used in spoken Korean. It's more informal than 'but' or 'however', and is used as 'and', 'by the way' depending on the situation.

근데 and 하지만 (BUT and HOWEVER) i'll give you an example, so it's up to you on how to understand the sentences. BUT 1. I will help you but I don't know how. 2. I love your sister but she hates me. 3. I love to eat cakes but my mom wont allow me to. HOWEVER 1. We will help him however he must help us too. so BUT and HOWEVER are the same ..