Hey Marianne,
If you just want to come for a year, it shouldn’t be a big deal to just come and overstay your tourist visa. There are plenty of Americans doing that right now in Spain! It’s very unlikely they would catch you. The only real risk is if you take a trip out of Spain and try to come back in (after the 90-day legal period has ended). Then there’s a chance they could study your passport, but it’s VERY unlikely they would do this if you’re American. It’s important to note, though, that they COULD…
As for the one-way ticket thing, me and plenty of other Americans have come to Spain on one-way tickets. I only know of one person who was asked to show a return ticket, and that was a Brazilian guy. The worst that could happen if they *do* ask you for one, and you don’t have one, is that they could make you buy one right then and there. So far, I don’t know of anyone who’s had this happen, but you never know… Sometimes, anyways, it was actually cheaper for me to buy a round trip ticket than a one-way ticket, and I just didn’t use the return leg.
Supposedly they can fine you for overstaying, but, again, I don’t know of anyone who has had to pay a fine. I was actually caught once when leaving Spain. I had been in the country for almost two years, and the US Airways security representative was the one who noticed it on my passport. She brought over the police, they questioned me for a while, and I basically just said, “Yes, I know I stayed too long, but I’m leaving now…” And they let me go.
What else? Oh, whether it affects you in the future. It could affect you if at some point in the future you want to apply for a work visa. When you apply for a work permit, you submit a copy of every page of your passport, so among other things they can check to see if you were ever living in Spain illegally. I met with three different immigration lawyers in Spain before applying for my work permit, and they all told me that having an illegal spate on your passport could very easily ruin your chances. The solution? If you know you’re going to be applying for a work visa in the future, lose your passport sometime after you return from Spain after living here illegally. 😊