Thanking someone properly

Muchísimas gracias

moo-CHEE-see-mahs GRAH-see-ahs — Thank you so much

"Muchísimas gracias por tu ayuda."
Thank you so much for your help.

"Muchísimas" is the superlative of "muchas" — reach for this over a plain "gracias" when someone has genuinely gone out of their way for you.

"What I say to my longtime students more often than they probably realize."— Graciela