No sooner

No sooner is used to show that one thing happens immediately after another thing. It is often used with the past perfect, and usually followed by than:

[event 1] They had no sooner arrived than [event 2] they were arguing.
[event 1] We had no sooner started cooking than [event 2] there was a power cut and we had no electricity.

When no sooner is used in front position, we invert the order of the auxiliary verb and subject. This is common in more formal and literary styles:

[event 1] No sooner had they started their walk than [event 2] it started to rain.