Did Nelson Mandela go to jail in 1990?

I knew from 7th grade that Mandela was imprisoned in the sixties and then released nintiesish, but on the answer key for my apush final review, the answer to "True or False - Nelson Mandela was sent to jail in 1990" was true. I asked my teacher, Bender, Dan Bender, The one and only famous Dan Bender, wtf was up with that (I told him he was wrong) and he alluded to some notion that Mandela was freed in '90, but if you look at it, he was really still in jail or sent back or something along those lines until later in the ninties... he said he'd address it and HE NEVER DID. Can/would anyone enlighten me please?