if a striker is offside but the linesman doesn't see it, does he score a goal?

Yes.

Inexplicably, Italy lost 3-0 to the Netherlands yesterday. I can't really talk about it much, as it is very painful, but I just want to point out that UEFA's explanation of van Nistelrooy's first, contentious goal (clearly offside) made absolutely no sense at all. The man sat there and recited the offside rule out of the rulebook and then gave an explanation so badly articulated that I can only assume that they wrote it to be so badly articulated in the hope that people will pretend they understood for fear of looking stupid.

But, guys. These are Italians, here. These are guys who have, in the past, insisted that their socks are too tight when failing to make a penalty shot. The whole world saw that van Nistelrooy's goal was offside; only the linesman didn't.

I really, really hope that linesman isn't planning on taking a vacation in Italy any time soon.