Thanks Khun Simon and Khun Pensive. Also, apologies to khun pensive. Was just trying to get more feedback.

Khun Nan, your example sentences were very helpful.
So it appears that when the particle combination NA SI is used in a reply to a question, it conveys a sense of
แน่นอน or
มันจะเป็นอะไรอย่างอื่นได้ยังไงล่ะ? to the answer.
When NA SI is used in a statement, not in reply to a question, it emphasizes more a feeling of surprise or disappointment.
pi_choen wrote:ไปกินข้าวกันเถอะ let's go eat ......what you think
Nan wrote:This is correct, but it's different to "ไปกินข้าวกันซิ let's go eat.".
For example;
A: ตอนนี้ว่างแล้ว ทำอะไรกันดีล่ะ
B: ไปกินข้าวกันสิ (ซิ)
I still wasn't sure of the distinction between B answering:
ไปกินข้าวกันสิ (
ซิ) or
ไปกินข้าวกันเถอะ.
Are both not denoting the imperative ie giving a command to eat?
I also seem to remember reading somewhere that the particle
ไง can be roughly equated to 'of course'
eg
In the above example
A:
เมียของผมไปไหนแล้ว? - Where's my wife gone?
Could B also answer:
ไปอยู่กับผัวเก่าของมันไง to mean, "Back with her old husband of course."
Or is this incorrect in Thai?
Thanks in advance.