If you can read Thai, you must know that ทร is /s/ such as ทราบ /sàab/ = to know, ทราย /saay/ = sand, etc. but sometimes you might notice that Thai people pronounce them as diphthong such as ทรัมเปต = trumpet or ทรู = true.
Originally, we don’t have diphthong ทร /tr/ sound in Thai phonology. All words with ทร were sounded out as /s/. At the beginning of Rattanakosin Period (1782 – present), we started to see the change.
อินทรา /in-tra/ = Indra, an Indian god; นิทรา /ní-tra/ = sleep and จันทรา /jan-tra/ = moon were exceptions, changed from /s/ to /tr/ sound in Rattanakosin Period.
It’s useful to learn that words with ทร usually are loanwords. If they are from Khmer or Pali-Sanskrit, we usually pronounce as one single sound /s/ except those 3 words above. If they are from English language such as trust, trumpet, tractor, etc., it’s a common practice to pronounce as /tr/.
Any other words with ทร that you know? 🙂