Pay
Pay back — To give someone back money that you owe them. (Can be separated by the person getting paid back.) When it’s written as one word, “payback” means revenge.
“Thanks for getting me lunch when I forgot my wallet at home! I’ll payyou back tomorrow.”
Pay for — This phrase can either mean to give someone money for a particular purpose (like paying for a new car), or to suffer because of something you did.
“He’ll pay for all the problems he caused me by being late today!”