The word used depends on the gender of the person who has spoken. For a man it is "krap" For a woman, it is "ka". So the Thai for hello is "sawatdee" and if said by a man, it becomes "sawatdee krap/kap" and for a woman, "sawatdee ka". Thankyou is "khopkhun"... "krap/kap" or "ka".