If you want to use the Thai words for thank you then you should add an extra word depending on whether you are male or female. A woman should say khop khun kha and a man should say khop khun khap (to be extra polite khap becomes khrap, but in everyday speech the latter isn’t used as much). It might seem confusing at first because there is no English equivalent for the words khap and kha. In effect they are similar to using intonation in English and convey feelings rather than meaning. They should also be used when saying hello or goodbye in Thai, which then becomes sawatdee khap (spoken by a man) or sawatdee kha (used by women).