Thai words only use tone marks when they deviate from the tonal rules of the consonant and vowel + ending particle. For example: วัน (day, pronounced wan) has no tone marks. It follows the tonal rules whereby mid consonant + short vowel + live ending (non P/T/K) = no tone. อ่าน (read, pronounced arn) has a tone mark. It deviates from tonal rules whereby mid consonant + long vowel + live ending (non P/T/K) = no tone. With the tone mark it now has a low tone. The Thai writing system is complex and confusing. Read here for more details on the classification ofThai consonants and how they define the tone rules.