Liliane Haegeman, Herman Wekker, The Syntax and Interpretation of Futurate Conditionals in English, Journal of Linguistics, Vol. 20, No. 1 (Mar., 1984), pp. 45-55 ...

Use the third conditional to imagine a different past. If I hadn't become an English teacher, I would've trained as a vet. If our train had been cancelled, we would've been late for the event.