I was tutoring a girl the other day when she accidentally spilled the glass of water she was drinking. She quickly cleaned it up and said:
Vamos a continuar como si nada hubiera pasado.
I asked her to repeat this a couple of times, since it was the first I had heard of it (and I knew it’d be good for her to know in English as well). This isn’t really an idiom, per say, because it actually translates quite literally. It means “Let’s continue as if nothing has happened” or “carry on as if nothing had happened.” Another way of expressing the same sentiment in English is “business as usual.”
“Tropecé cuando vi mi enamoramiento, y traté de actuar como si nada hubiera pasado.”
I tripped when I saw my crush, and I tried to act like nothing had happened.
“Mi novio sabía que yo no era feliz, pero él actuó como si nada hubiera pasado.”
My boyfriend knew that I wasn’t happy, but he acted as if nothing had happened.