Is it proper grammar statement: <br/>'I didn't under stand this (problem#1) OR this (problem #2)'.<br/><br/>If not, is it correct:<br/>'I didn't under stand this (problem#1) AND this (problem #2)'.<br/><br/>Does the Tense matter - is it true that:<br/>Present Tense - I DON'T understand this AND that.<br/>Past Tense - I DIDN'T understand neither of these problems.<br/><br/>Thank you