"If you have moved your king, you can'y castle".That's what GM Seirawan says.
1)What tense is he refering to in his If-clause?
2)Why doesn't he say:"if you move your king you cannot castle"?, refering to a future tense?
When we refer to future time in an if-clause, it is inappropriate to use will with the meaning of certainty in a situation in which there is uncertainty. We therefore use an unmarked (present) tense form, simple, progressive or perfect.