Have a checkbox make another field required

Hello, I cannot figure out how to make a drop-down list required and reset if a check box is checked. If unchecked, it changes to read-only with a defined answer.

Checkbox name - CBDrive

Dropdown name - DDInter

If CBDrive is checked then DDInter is required and reset to empty,

If CBDrive is unchecked, then DDInter read-only and value = "N/A".

Thanks for any assistance.