user1105: Yesterday I was playing with a friend, I was almost out of time, he checked me, didn't say anything, I didn't see the check as i was in a hurry to make my move. And I was mated.
Computers don't let you play any move when you're in check. Therefore in reality, are you allowed to make a move when you're in check? or you cannot make any move? I mean should you undo any move that you do by mistake or not? And should your opponent warn you about being checked or not?

user14143: Lynob, announcing check is not required in the USCF Official Rules or FIDE Laws of Chess. In fact, there is a statement specifying that it is not an obligation. The requirement was abandoned in the early 20th century. Before that, players would even say "garde" to alert an opponent that his queen was being attacked. How polite can you get? After all, it is purportedly a spirited competition.
That said, old school or beginners alike may be hard put to suppress the impulse to announce check, but if said frequently an opponent may request intervention by a TD to admonish the offending player against verbal distraction. It's not prohibited in friendly games but even there it's no longer a vital fixture. Nor is it an abandonment of politeness so much as a logical extension of over-the-board etiquette. We don't want to be distracted ourselves and so we avoid distracting our opponents.
It might even be taken as a tad insulting considering that there are only two participants in a game that takes place on a playing field just 20 inches square. Given that check is the most serious forcing move that can possibly occur, it's a given that both players are expected to be paying keen attention to what's happening right before their eyes. One may see "deeper" than another, but they both see the same physical thing!
I admit, nonetheless, that players of any skill level do often enough fail to notice they are in check, so rapt are they in contemplation of potential future activity. In a rated tournament, the conventions require that if a player makes an illegal move instead of resolving check and then punches the clock, the opponent may immediately press his own clock button to restart the player's time and then politely inform him that he has overlooked check. The touched piece illegally moved would then be required, if possible, to intervene with the check. If not possible, there is no penalty and any move that alleviates check must be made before the player can again punch the clock.

user3270: In a tournament game, you are not supposed to speak at all other than to offer a draw or adjust a piece. This includes saying check. This tends to seep into casual games among more experienced players (I haven't said check in years). 
For players that do not compete in tournaments at all, I don't think that there is any obligation to point it out.
When your opponent makes an illegal move, though, as it sounds like you did after missing the check, it has to be pointed out. You can't just follow it up with a mate immediately. In blitz, making an illegal move means a loss. In a longer game, a minor time adjustment.

1105: <p>Yesterday I was playing with a friend, I was almost out of time, he checked me, didn't say anything, I didn't see the check as i was in a hurry to make my move. And I was mated.</p>

<p>Computers don't let you play any move when you're in check. Therefore in reality, are you allowed to make a move when you're in check? or you cannot make any move? I mean should you undo any move that you do by mistake or not? And should your opponent warn you about being checked or not?</p>
 14143: <p>Lynob, announcing check is not required in the USCF Official Rules or FIDE Laws of Chess. In fact, there is a statement specifying that it is not an obligation. The requirement was abandoned in the early 20th century. Before that, players would even say "garde" to alert an opponent that his queen was being attacked. How polite can you get? After all, it is purportedly a spirited competition.</p>

<p>That said, old school or beginners alike may be hard put to suppress the impulse to announce check, but if said frequently an opponent may request intervention by a TD to admonish the offending player against verbal distraction. It's not prohibited in friendly games but even there it's no longer a vital fixture. Nor is it an abandonment of politeness so much as a logical extension of over-the-board etiquette. We don't want to be distracted ourselves and so we avoid distracting our opponents.</p>

<p>It might even be taken as a tad insulting considering that there are only two participants in a game that takes place on a playing field just 20 inches square. Given that check is the most serious forcing move that can possibly occur, it's a given that both players are expected to be paying keen attention to what's happening right before their eyes. One may see "deeper" than another, but they both see the same physical thing!</p>

<p>I admit, nonetheless, that players of any skill level do often enough fail to notice they are in check, so rapt are they in contemplation of potential future activity. In a rated tournament, the conventions require that if a player makes an illegal move instead of resolving check and then punches the clock, the opponent may immediately press his own clock button to restart the player's time and then politely inform him that he has overlooked check. The touched piece illegally moved would then be required, if possible, to intervene with the check. If not possible, there is no penalty and any move that alleviates check must be made before the player can again punch the clock.</p>
 3270: <p>In a tournament game, you are not supposed to speak at all other than to offer a draw or adjust a piece. This includes saying check. This tends to seep into casual games among more experienced players (I haven't said check in years). </p>

<p>For players that do not compete in tournaments at all, I don't think that there is any obligation to point it out.</p>

<p>When your opponent makes an illegal move, though, as it sounds like you did after missing the check, it has to be pointed out. You can't just follow it up with a mate immediately. In blitz, making an illegal move means a loss. In a longer game, a minor time adjustment.</p>
