Thai syllables can only end in eight sounds. Whatever letter is written, at the end of a syllable its sound collapses to one of the eight — ส sounds like s at the start of a word but like t at the end. How a syllable ends decides whether it is livei or deadi — the heart of the tone rules later.
Three are stopsi (the syllable is dead), five are sonorantsi (the syllable is live). In our transliteration a final y shows as -i (khwaai) and a final w as -o (khâao) — that's just how the glidei reads on paper; the mouth is doing y and w.
Letters you'll never see ending a syllable: ฉ ผ ฝ ห ฮ อ.
A small mark called gaa-ran (การันต์) written over a letter kills it — the letter is written but not spoken. Common in words borrowed from other languages: ยักษ์ yák (giant) — the ษ is silent.