i have an old iMac and want to get a 13" MacBook Pro. Can I use the screen on the iMac with the laptop?

You cannot use an iMac as a screen with a 2020 or later Mac.

BTW, adding an external display is far less trouble, much more reliable and does not require very expensive cabling than using TDM. I would recommend looking for an external display, you can find some decent ones in the $100 range.

You're welcome.