Definition
Used to indicate the time when something may or may not occur.
如果和當…時
We're not sure if we'll need extra help, but if and when we do, we'll let you know as soon as possible.
我們不確定是否需要額外幫助，但如果需要的話，我們會盡快通知你。
This project will only start if and when we get the approvals and sufficient funding.
這個項目只有在我們獲得批准和足夠資金的情況下才會啟動。
I will only raise an objection if and when I feel it's necessary.
我只有在覺得有必要的時候才會提出異議。