The consonants - 'small' (in red) and 'big' (in blue). When a vowel follows a 'small' consonant, the vowel has a different sound to when it follows a 'big' consonant. Do the maths - it makes for a lot of sounds!
Another interesting thing we now learn is that what is written (for example, in a dictionary) very often is not what is pronounced, as the Khmer love dropping off the last sound. So, when the dictionary says 'mork' the pronounciation is 'mor'. That goes a way to explaining why what we have been learning does not always line up with the pocket dictionary!