Polite ending to elders: … ạ

Ạ at the end of a sentence adds respect and nothing else. Use it with someone older or senior to you: a teacher, a friend's parents, an older neighbour. Em cảm ơn chị ạ is a more respectful "thank you" than Em cảm ơn chị.

It goes last, after the address word: Con chào cô ạ. It also makes a question more polite: Chị ơi, cái này bao nhiêu tiền ạ? The meaning stays the same with or without it.

In Saigon, polite speech rests mostly on dạ at the start and the address word at the end: Dạ, cảm ơn chị. Ạ is common too, especially from children to adults and in careful speech, often together with dạ: Dạ, em hiểu rồi ạ. In Hà Nội you'll hear ạ much more often, as in Vâng ạ.

Don't use ạ with friends or with anyone younger; it sounds stiff, or as if you're teasing. Service is the exception: staff in shops and cafés often use it with customers of any age. And keep it apart from à, with the falling tone, which checks something you've just heard: Chị về rồi à? You're back? Ạ, with the dot below, doesn't make a question by itself, but to someone older the same check often ends in ạ instead: Chị về rồi ạ?