As soon as: ngay khi

Ngay khi means "as soon as", "the moment": Ngay khi về tới nhà, em sẽ gọi cho anh. As soon as I get home, I'll call you. Ngay is "right, immediately" and khi is "when", so it's literally "right when".

The ngay khi part usually comes first, followed by a comma. It can also come second, which is common in work messages: Em sẽ báo chị ngay khi có kết quả. I'll let you know as soon as I have the results. In writing, ngay sau khi means "right after".

In relaxed speech you'll often hear vừa … là instead, with vừa before the first verb and là before the second: Vừa ra khỏi nhà là trời mưa. The moment I stepped out, it started to rain. Liền (right away; in Hà Nội ngay) often ends the second half: Vừa về tới nhà là em gọi anh liền.