English carries politeness and attitude in intonation: the difference between 'sit down' and 'sit down?' and 'sit down!' is melody. Thai cannot do that: melody is already fully occupied marking which word you said. A rising contour is not a question, it is a different lexical item.
So the work has to go somewhere else, and it goes to a set of small words at the end of the sentence. They are not filler and they are not optional decoration. They are the part of the grammar English puts in the voice.
- ไปนะpaj ná
I'm off, all right?
นะ softens, seeks agreement
- ไปสิpaj sì
go on, go
สิ pushes, encourages
- ไปเลยpaj lɤːj
go right ahead
เลย: immediately, without hesitation
ครับ and ค่ะ are not optional
These two mark the speaker, not the listener: ครับ if you are male, ค่ะ if you are female. Dropping them does not sound casual, it sounds curt, closer to answering a stranger with a grunt than with a word.
Get them automatic in the first fortnight and you will sound polite for six months without thinking about it. It is the highest return on effort available to you in week one.