How modern Burmese got two grammars

First there were the Buddhist scriptures in Pali, with Burmese interlinear translation, with conventional markers of number, tense, case, and mood corresponding to Pali ones, just like the 1SG and ACC and PL that linguists use nowadays.

Next were the same texts, but with the Pali original left out: a Burmese relexification of Pali, with conventional markers aforesaid. This is called "Nissaya Burmese": Burmese surface representations, Pali morphology and syntax.

Next were original works written directly in Nissaya Burmese.

Then came more original works in modified Nissaya, with some of the Pali markers left out. The more Pali-esque, the more high-toned the work was considered to be.

Next came writing in plain Burmese, but heavily influenced by Nissaya conventions. This is roughly the level of newspaper writing in Burmese today.

Next came elegant spoken Burmese, which was native morphosyntax with many conventions taken from prose style, which was itself a mixture of native and Nissaya.

Finally, colloquial spoken Burmese picked up many of these conventions as well.

It seems that one cannot fully describe the linguistic habits of the Burmese without using two sets of grammatical rules, one just like the grammar of Pali, the other more characteristic of colloquial Burmese. [Burling]

So are the most Nissaya-ish varieties really Burmese, or are they really Pali? Well, the grammar is surely Pali, but they are connected by an unbroken chain of mutually intelligible 'lects to colloquial Burmese, whereas Pali itself is completely shut off by an impermeable lexical barrier. (Burmese has borrowed many Pali morphemes, but not so much that Pali is even vaguely intelligible without learning it.)

Note that the Pali influence in Burmese has been going on for a thousand years: Burmese itself has evolved considerably from the form in which the interlinear scriptures were first recorded. Pali morphosyntax is now so deeply intertwingled in Burmese that we can only recover a "pure native" grammar from comparative Tibeto-Burman considerations.

References:

John Okell, "Nissaya Burmese", Indo-Pacific Linguistic Studies (Lingua 15), (Amsterdam: North Holland Publishing Company, 1965) [the original]

John Okell, "Nissaya Burmese", Journal of the Burma Research Society, 50 (1967) pp. 95-123 [expanded version]

Robbins Burling, Man's Many Voices (Holt, Rinehart and Winston, 1970), pp. 180-83 [the secondary source that I actually read]