Is thank you spelled with or without a hyphen?
Both spellings are correct, but they have different uses.
Without a hyphen, thank you is a verb phrase. We use this phrase to show our gratitude to someone:
Thank you, Alan, for the lovely lawn mower!
I want to thank you, Ms. Krishna, for your valuable help.
With a hyphen, thank-you is either a noun or an adjective. As a noun, it means "a message of thanks":
Mark sent Joan a thank-you for the lava lamp she gave him.
As an adjective, thank-you is used before a noun and means "expressing thanks":
I received a thank-you card from my stock broker.