"check"

Hi,

Sounds like a stupid question, but I am not sure of the answer. 

If you are playing against someone in person, do you have to say "check" if you put them in check, and if that is the case, and you don't, can the other player ignore it and move something else (even putting me in check, whilst he is still in check)?

 There is no requirement for you to announce check. The other player cannot ignore the fact that he/she is in check.  He/she must make a legal move.  As to whether or not it's good manners to announce check there are two groups of thought and there was a forum thread on the subject.  My opinion is that most tournament and league players do NOT announce check and consider it bad manners.  Others, and there are many, were brought up to do so and consider it polite to warn the opponent.

http://www.chess.com/forum/view/general/announcing-check?page=1

Saying check is a courtesy. It's polite, and in friendly play it's often standard procedure. It is not required to announce check, however. Whether or not check is announced, the player in check has to protect his king.

Thought so. Thanks for clearing that up, yet another dubious rule he has tried to use on me. I guess he must be desperate for a win :)

Saying check in rated game is often considered rude above Class D. The reason is that any half-decent player would notice he's in check and secondly, it disturbs other players in the tournamnet hall.

Most consider it a polite warning, however some players (oh so sensitive) can feel insulted, of course they saw that check!