Thai has many loanwords from Sanskrit and Pali. Often there are two
different froms, one from Pali and one from Sanskrit, often with
slightly diffeent meanings. One striking feature is that the form
said to come from Pali appears to come from a word with /b/ while
the other form has /w/ (Modern Thai does not distinguish /v/
and /w/.) A Pali /b/ (or a /b/ substituted for a /v/) appears in
modern Thai as /ph/ at the start of a Thai syllable, and appears
as /p/ at the end of a Thai syllable. The spelling is the same.
What is the origin of this substitution? There are many such
doublets, such as 'wihaan' and 'phihaan' from 'vihaara'. In a few
cases the change of 'v' to 'b' does seem to have happened in Pali,
as in Thai 'theep' from Pali 'deba', but 'theew' from
Sanskrit 'deva'. I am not entirely certain that is only Pali words
that are changed this way (/v/ > /b/). It seems to have affected
some loans from Sanskrit, but it may be that these words are Pali
words whose spelling has been Sanskritised.
As recently as the 16th century AD, Thai seems to have
distinguished /b/, /v/ and /w/, which are now /ph/, /f/ and /w/
respectively. I am not aware of any Thai words in which /f/
corresponds to Pali /v/, so it does not seem likely that if results
from an uncertainty in how to say Pali /v/ in Thai.
I gather that the 'Old Sumatran' script of Sri Javaya did not
distinguish /v/ and /b/. It used the letter <v> for both, but the
pronunciation in modern Malay is /b/. I have heard that, more
generally, many Pali manuscripts do not distinguish /b/ and /v/, but
that exhausts my knowledge of the merger of /b/ and /v/.
Pali written in the Thai alphabet now appears to distinguish /b/
and /v/, but I have only one book containing Pali written in Thai to
go by. This book has what seems to be an unusual spelling for
Pali /a/. Instead of using an inherent vowel, it writes it with an
explicit vowel, and does not mark the lack of a vowel. I must say
it makes it easier than looking for a subtle pinthu when syllabifing
the text!
I would appreciate any light on this subject. I have tried various
forums, but without success.