When do you hyphenate the word thank you?

You don't hyphenate "thank you" in most cases. It's always two words, except in a few very specific circumstances:

* When it's used as an adjective: You might hyphenate "thank-you" if it's describing something, like a "thank-you note" or a "thank-you speech."

* When it's part of a compound word: You might see "thank-you" hyphenated as part of a compound noun, like "thank-you-giver". However, these are uncommon.

In general, stick with "thank you" as two separate words.