Where a word starts with a consonant cluster which can not be pronounced in Thai a vowel must be inserted between the consonants. Usually that vowel is an unstressed /a/ (no glottal stop) [see footnote]. So, for example , แสดง (to show) is pronounced /saˑdɛɛŋ/, and อดีต (the past) is pronounced /aˑdìit/.
However, if the first consonant of the second syllable is one of the sonorants ง, น, ม, ย, ร, ล or ว, the tone of the second syllable is determined by the class of the initial consonant. So, for example, with ผลิต (to produce) ผ is high class, ล is a sonorant, so ลิต is pronounced according to the rule for high class, dead syllables, i.e. low tone. Consequently ผลิต is pronounced /phaˑlìt/.
Let's take another example, ถนน (road). /a/ is inserted after ถ, ถ is high class, so นน is pronounced according to the high class, live syllable tone rule, i.e. rising tone. This gives us: /thaˑnǒn/.
Try reading the following words, paying particular attention to the tone of the second syllable.
The position of leading vowels can be inconsistent. Usually the vowel is written before the cluster, e.g. เสมอ (/saˑmə̌ə/, always) and แสลงใจ (/saˑlɛ̌ɛŋˑcay/, to feel hurt). However, in some words it's written within the cluster, e.g. ขโมย (/khaˑmooy/, thief), นโยบาย (/naˑyooˑbaay/, policy). In these cases the class of the initial consonant is not used for the second syllable.
In compound words, the unstressed /a/ can occur within the word. For example, ลืมสนิท (to forget completely) is pronounced /lʉʉmˑsaˑnìt/, and ผลสรุป (conclusion) is pronounced /phǒnˑsaˑrùp/.
Occasionally there are two (or more) consecutive syllables with unstressed /a/. Here are some of the commonest ones.
Now try reading these words:
Many sources suggest that the /a/ has a high or low tone, and ends with a glottal stop. This, however, is not true in normal speech. To quote one authority:
"Only in artificial 'dictation' style do they close with a glottal final.
Only in dictation style, moreover, do they bear the phonemic tone we would expect from their spelling."
Bee, Peter J. (1975) "Restricted Phonology in Thai Linker Syllables" in "Studies in Tai
Linguistics in Honor of William J. Gedney", Bangkok: CIEL, pp. 13−42.