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

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

You're welcome.