How do you say 10 45 in spanish?

"Son las once menos quince" or "son las once menos cuarto" (it is quarter to 11). You could also say "son las dies y cuartena y cinco" (which means it is 10:45).

You could also add "de la mañana" (in the morning), "de la tarde" (in the afternoon), or "de la noche" (at night) to the end to make it more correct.