But for is used to introduce the reason why something didn’t happen.
But for the traffic jam, I would have come in time.
Если бы не пробка на дороге, я бы пришел во время.
(The traffic was very heavy. If it weren’t for the traffic, I’d have come in time.)
Схема составления предложения, используя оборот But for…
But for + noun/gerund (V+ing)
subject + would/could/might + have + past participle
They would have all perished, but for the quick thinking of the driver. (=… if it hadn’t been for the quick thinking of the driver)
But for the flight cancellation, I could have arrived at the conference.
Make up your own sentence or describe a real situation using but for …