I has a similar problem when learning Spanish - I had no idea that the feminine wouldn't just be the same word with an "a" on the end. In general though, I found Spanish people amused by my attempts and impressed that I was making an effort (I'm sure that I got away with a lot of incorrect words).
Its funny how difficult it is for people to recognise a word that is very nearly right. One of my friends speaks the most beautiful sounding French (I don't speak good French so I can't say how good it is but to me, she sounds like a very elegant French speaker), yet when she live in Paris for a year as a student people regularly blanked her out as if she was saying something very strange. In most cases she was very nearly right. She wondered if it was because of the nauture of the French language that it is hard to work out an "almost correct" word or because Parisiennes don't want to try to understand.
