But: nhưng

Nhưng means "but". It joins two parts that pull against each other: Món này ngon nhưng hơi mắc. This dish is tasty but a bit expensive (in Hà Nội, hơi đắt). It goes at the start of the second part, just like "but", and a comma before it is optional.

When the second part is about the same person or thing, there's no need to repeat it: Phòng nhỏ nhưng sạch. The room is small but clean.

In speech people very often say nhưng mà, which means the same and sounds more conversational: Em muốn đi lắm, nhưng mà em bận. It can also open a sentence, to answer or object: Nhưng mà em không biết đường. But I don't know the way.

Mà alone also means "but" in speech, often with a note of surprise: Rẻ mà ngon. Cheap, and yet good.

One difference from English: after mặc dù or tuy (although), Vietnamese often still adds nhưng in the second half: Mặc dù mệt nhưng em vẫn đi làm. English drops the "but": although I was tired, I still went to work.