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