Do you know how to read/write in Thai? because it might be easier to explain if you do. Generally, Thai words follow this pattern: First, you have to be able to recognize the three classes of Thai consonants. High, Mid, Low. Then, if there is no tone mark, determine if the syllable is "live" or "dead" If there is a tone mark, then depending on what "class" the first consonant is will determine the tone. http://www.thai-language.com/ref/tonerules This website is very helpful in explaining it. Its complicated, but once you have it memorized it is not so difficult. Good luck.

What's the law of Thai tone ？ 漢語有 “平上去入”四声 4個聲調和漢語對照就如下 平=สามัญ 上=ตรี 去=เอก 入=จัตวา 泰語也一樣有， 可除此之外泰語里頭還多出了一個聲調，那就是 เสียงโท 不過要注意一點。 所有泰語字母本生還嚴格的分 （高，中，低）聲。 所以這個音節不都可以直接用的上。