In my previous post, I discussed the pronunciation of the vowel in the second syllable of tasik ('lake'), specifically that it may be pronounced as [i] or [e]. In a similar fashion, the vowel at the end of terima kasih ('thank you') can often be heard as [e].
However it is not true that all vowels in the final syllable of words in Malay can vary in this way. For example, the second vowel in boleh ('can') is [e] and cannot be [i], and similarly the second vowel in oleh ('by') must be [e] and cannot be [i]. Why is this?
The rule is this: if the vowel in the first syllable is [e] or [o], then the second syllable cannot have a more close vowel. This is known as vowel harmony: the vowels in successive syllables of a word may influence each other. However, the vowel in the first syllable of both tasik and kasih is [a], so these words are not subject to this rule of vowel harmony.
For more information on vowel harmony and other aspects of Malay phonology, see my Pronunciation of Malay website, which provides an on-line version of my paper on Malay, written together with my UBD colleague, Adrian Clynes.