should/would/could've +pp（仮定法過去完了）

If you had trained more thoroughly, you could've completed the marathon.

thoroughly ソﾛｧ-ﾘ- adv)徹底的に

complete ｺﾝﾌﾟリ-ﾄ v)完全･完了する、仕上げる、申し分ないものにする

Had I known/I wish I had known/If I had known it would/was going to rain today, I would've brought my umbrella with me.

Had I known ( If の省略形）

I wish I had known 知っていればよかったのにな 

I wish I had known you were a good teacher, I could've taken your lesson more.