Go ahead, just keep on: cứ + verb

Cứ before a verb tells someone to go ahead and not hold back: Có gì em cứ hỏi anh. If anything comes up, just ask me. It reassures: there's no need to worry, wait or be polite about it. It goes after the person and before the verb, and đi or nha can follow at the end.

You'll hear it most in a few friendly phrases. Cứ tự nhiên is "make yourself at home" or "help yourself". Anh cứ để đó is "just leave it", often with an offer: em làm cho, I'll do it. To a guest who starts to get up, Chị cứ ngồi đi means "please, stay where you are".

Cứ also says that something keeps happening, often when it's annoying: Trời cứ mưa. It just keeps raining. In Saigon, hoài at the end adds "over and over" (in Hà Nội, mãi): Thằng nhỏ cứ khóc hoài. The little boy keeps on crying.

Cứ gives permission or encouragement. Hãy gives an instruction and sounds formal, and đi at the end urges someone on.