Oh, by the way: À, … at the start

À at the start of a sentence is the "oh" of a thought that has just come to you: À, mai chị rảnh không? Oh, are you free tomorrow? It often brings up something new, like "oh, by the way", and there's a short pause after it, which the comma shows.

You'll also hear it when something becomes clear: À, hiểu rồi. Ah, I get it. À, ra vậy. Oh, so that's why. When you remember something you nearly forgot, it's À quên: À quên, chìa khóa em để trên bàn nha. Oh, I forgot: I left the key on the table.

À mà (oh, and) brings in a second point: À mà, tối nay ai nấu cơm? À, nghe nói … (oh, I heard …) opens a bit of news. All of these are casual, fine with friends, family and colleagues.

You know à at the end of a sentence, where it checks something you've just heard: Anh là người Úc à? At the start it asks nothing; it only shows that the thought is new. Don't confuse either with ạ, with the dot below, the polite ending for older people.