some verbs (like in your sentence "nie wiem") have the -em ending,
(ja) wiem, rozumiem (but in 3rd person it's more regular "(on/ona/ono) wie, rozumie"), many people with bad education say "(ja) wię, and especially rozumię", but don't follow them, it's incorrect :)
Hmm, that is a good heads up. :) Are there any set rules for verbs that take the -em ending, or do these have to be memorized?