The first sentence can mean that the person is still married. The second sentence suggests that the person was married in the past but is now divorced or widowed.

The first sentence implies the person is still married. The second sentence is what you would say if the person were no longer married

As above, BUT this article may help to explain why......it seems complicated, but should help you!:) Basically, we match the tense to 'didn't' = past, because the emphasis of the statement is that you didn't KNOW.....The second sentence uses the past perfect - which firmly places it in the past = dead and buried = no longer married! :) http://english.stackexchange.com/questions/149120/i-didnt-know-you-liked-her-or-i-didnt-know-you-like-her