~ないうちに (before it happens)

I met my now-husband over email, and once we'd started dating through some weird twist of fate, before I even knew it, we'd decided to get married within less than a month.

Before I could say a word, he hung up.

Don't get off before the train has stopped.

Let's leave before the sun rises.

Write down his phone number before you forget.

I must write down his address before I forget.

You'd better write it down before you forget.

Before I forget, let me just mention...

She cleaned the room before the guests arrived.

She tidied up the room before the guests arrived.

Jot that down before you forget.

Do it right away before you forget.