Whenever (every time): hễ (cứ) … là

Hễ … là means "whenever, every time": as soon as the first thing happens, the second follows, without fail. Hễ trời mưa là đường này ngập, whenever it rains, this street floods.

Hễ goes at the start of the first half, before or after the person: Hễ thấy kem là nó đòi mua or Nó hễ thấy kem là đòi mua. The second half usually starts with là, or with thì, which sounds a little more neutral. Without one of them the sentence can sound unfinished, so keep the pair while you're learning. Adding cứ makes it stronger, every single time: Hễ cứ uống cà phê buổi chiều là tối em mất ngủ.

It's for habits, patterns and things that happen predictably, and it often carries a small sigh, as if you've seen it a hundred times. It also works for an offer that stands at any time: Hễ có chuyện gì thì gọi chị nha, if anything at all comes up, call me.

The close everyday neighbours are cứ … là (Cứ tới Tết là kẹt xe, every Tết the traffic jams up) and mỗi lần … là (each time). Plain khi (when) only says when something happens; hễ says it happens every time.