Most Thai sounds are easy for English speakers. A handful aren't, and they're the ones that turn one word into another. Here they are, one at a time.
The short answer: four things make Thai pronunciation different from English: tones (five of them), aspiration (whether a consonant has a puff of air: ไก่ kài chicken vs ไข่ khài egg), vowel length (เข้า khâo enter vs ข้าว khâaw rice), and stopped final consonants (a -k, -t or -p that closes the sound without releasing it). Each one can change what a word means.
Every Thai syllable carries one of five tones: mid, low, falling, high or rising. The tone is part of the word. มา maa is come, ม้า máa is horse, หมา mǎa is dog. Tones have their own guide, with a free test that draws your voice next to a native one: Thai tones: the five tones and how to say them.
Hold your hand in front of your mouth and say kite. You'll feel a puff of air after the k. Now say skip. The k there has almost none. English never uses that difference to tell words apart. Thai does.
The same split runs through three pairs of sounds. In romanised Thai, an h after the letter means "with a puff":
| No puff | With a puff |
|---|---|
| k · ไก่ kài, chicken | kh · ไข่ khài, egg |
| dt · ตา dtaa, eye | th · ที่ thîi, at |
| bp · ปลา bplaa, fish | ph · พูด phûut, to speak |
th is never the English "th". ที่ thîi starts with a t and a puff of air, not the sound in think. Likewise ph is a p with a puff, never an f. That's why ผัด phàt, stir-fry, is not "fat".
dt and bp are the no-puff versions of t and p. English has them, but only after an s: the t in stop, the p in spin. At the start of a word they sound halfway to a d or a b, which is why they're written with two letters. ตา dtaa, eye. ปลา bplaa, fish.
Thai also has a real d and b, as in เด็ก dèk (child) and บ้าน bâan (home), so the difference matters.
English has the ng sound only at the end of words: sing, long. Thai uses it at the start too: งู nguu, snake. Say singing slowly, stop just before the second "-ing" and hold the ng. That's where the word starts.
A Thai vowel is either short or long, and the length changes the word. Romanised Thai doubles the letter for a long vowel.
The long vowel isn't louder or higher. It's just held for longer, roughly twice as long. Clip it short and you're asking to "enter" when you meant rice.
A Thai word can end in a -k, -t or -p, but the sound is stopped, not released. Your mouth moves into position and cuts the air off, with no little puff at the end. English speakers tend to say ผัด phàt with a crisp t at the end. A Thai speaker closes the t and stops. เด็ก dèk, child, ends the same way: the k is there, but you barely hear it.
That's also why these endings are written -k, -t, -p and never -g, -d, -b. Whatever the Thai letter, a stopped ending comes out as one of those three.
Tones get the attention, but aspiration and vowel length cause just as many misunderstandings, because English speakers can't hear the difference until someone points it out.
kh has a puff of air after it; k doesn't. ไก่ kài is chicken, ไข่ khài is egg.
Most of the sounds are easy. The hard part is a small set of differences English never uses: puff or no puff, long or short, and pitch. Once you know to listen for them, you start hearing them everywhere.
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