"Good thing I didn't go"

You just have a past form of verb do ( didn't) so the other verb needs to be in present. That means that the answer '' didn't came'' is incorrect. I would say '' Good thing I didn't go ''.

"Good thing I didn't came" is not possible. "Do"+a verb here is a modal auxiliary (helping verb). When using a modal auxiliary, you must use the uninflected (unchanged) form of the verb. Good thing I didn't came = Incorrect Good thing I didn't come= Correct Good thing I didn't went= Incorrect Good thing I didn't go= Correct In your example, either "Good thing I didn't come" or "Good thing I didn't go" are both possible options, meaning essentially the same thing.