French negates the content of the obligation: il ne faut pas y aller means you are obliged not to go. Thai negates the obligation itself: ไม่ + ต้อง says there is no obligation in the first place.
The two readings are opposites, and the mistake runs in both directions. French speakers say ไม่ต้อง meaning 'you mustn't' and are heard as 'no need'; they hear ไม่ต้อง as a prohibition and feel told off.
- ต้องจ่ายtɔ̂ŋ tɕàːj
you have to pay
- ไม่ต้องจ่ายmâj tɔ̂ŋ tɕàːj
you don't have to pay
not 'you must not pay'
- ต้องไม่ไปtɔ̂ŋ mâj paj
must not go
same two words, other order, opposite sense
The words that do forbid
ห้าม is the one you will read: it is what is printed on signs, and it forbids in the abstract, to everyone.
อย่า is the one you will hear: it is spoken, aimed at the person in front of you, and it is an instruction rather than a regulation.
- ห้ามสูบบุหรี่hâːm sùːp bù.rìː
no smoking
the sign
- อย่าไปjàː paj
don't go
said to you, now
Why it is the most useful refusal you have
ไม่ต้อง on its own declines without using the word no. The bag at the 7-Eleven, the second helping, the offer of a lift: it says the obligation does not exist rather than that you are refusing the person.
That distinction is the whole of Thai polite refusal, and this is the cheapest way into it. It pairs naturally with ขอบคุณ and needs nothing else after it.
- ไม่ต้องmâj tɔ̂ŋ
no need; don't bother
- ไม่ต้องครับmâj tɔ̂ŋ kʰráp
no thank you
polite, complete, and not a no