Ever try to order food in a Thai restaurant only to be completely and utterly misunderstood, or even worse, ignored? Perhaps it's because you just came in from Khao San Road and are not wearing shoes and shirt. But in most cases it's probably because you're not pronouncing the names of Thai dishes correctly. Thai is a picky language, and a misdirected tone, a shortened vowel or improperly articulated consonant can mean the difference between a hot meal and yet another bag of "Thai Basil" chips at 7-11. And not only are Thai words hard to pronounce, but,there is no commonly accepted method of transliteration from Thai to English, so a dish you saw spelled one way on a menu last week, could very well be spelled differently in the next place. To help you understand and make yourself understood, I've put together a basic guide to Thai pronunciation and transliteration.
I sometimes hear non-Thais pronounce the first consonant of the second word like our t in English (ie aspirated, with a puff of air). Generally, if you see something written with a t, it is probably referring to the unaspirated t sound (somewhere between a t and a d, and similar to the t in the English word "stand"). On the other hand, if you see a Thai word with th, it is never pronounced like the th in the English words "thin" or "this", but is rather the aspirated (accompanied by a puff of air) t sound. The same goes for k, kh, p and ph. Sometimes the unaspirated t is also written as dt, which personally I find confusing.
Phat thai or phad thai?
In Thai there is no difference between a final t or d sound (or ph, p or b for that matter). However do keep in mind that ph is the aspirated p sound, and is not equivalent to f.
Larb vs. larp vs. laab vs. laap vs. lahp
The length of the vowel sound is very important in Thai, and to convey a long vowel sound, English transliteration often sticks an r (or sometimes an h) into the mix. Myself, I prefer just to double the vowel, as the Thai long vowel sound is not exactly like the English r sound. Regarding the words above, there is no "correct" spelling--just remember to pronounce a long vowel sound when you say the name of the dish.
Some sounds simply don't exist in English
Obviously transliteration has its limits, particularly in the lack of tones (a whole other can of worms that I won't get into here) as well as the various sounds that are common in Thai, but that aren't found in English. A couple examples of this are the eu sound, found in the word for "one" (neung), which is made by smiling (as opposed to rounding the lips). Another sound that doesn't have a real equivalent, and tends to be written any number of ways is the aw (like in the English word "law") sound. Sometimes you'll see this transcribed as o or or.
l and r
The written Thai language distinguishes between r and l, but in everyday speech, the r is almost always pronounced like an l (in Laos they've done away with the r sound altogether). This is why the dish raad naa, when said by a cook on a street corner in Bangkok, is going to sound more like laad naa.
A few words and phrases to help you along when ordering:
mii ... mai? Do you have ...?
mai kin ... I don't eat ...
neua sat meat
khawp khun thanks
thao rai How much? (the easiest way of asking for the bill)