I believe there are 5 different levels of spoken Thai, from the market woman to royals and elite. I speak kind of fluently basic Thai, but once visited a theatre show at the university and was suprised I hardly understood anything of it. Anyone here who can explain the system of language levels?

Is it true that spoken and written Thai have very different origins, so historically do not belong together?

For exactly the same reasons as there are many "different levels" of spoken English in the U.K.

Cheers.

It's mostly the vocabulary, of course.

Officially there's a completely different set of words for everything for his majesty.

In the written form you are expected to deploy the proper vocabulary.

So university play will have that written out and spoken.

But an average Thai would be at a loss if expected to use it on the fly.

In normal society, expect to learn the polite form and the common form of everything.

Use the polite form as a general rule when you write as well as when addressing a person of respectability.

ขี้ is shit and is rude when said to your doctor, for instance.

Say อุจจาระ instead.

The act of moving bowel is also ขื้, but say to your doctor ถ่าย

However, it's alright to say ขี้หมา dog poo to your doctor, because saying dog feces/faeces is like elevating the dog to the status of people.

To really master any language to the literary level is not a birthright.