<span style="color:#ff0000;">1-I did not understand one bit of it.</span><br/><span style="color:#ff0000;">2-I did not understand a bit of it.</span><br/><br/><span style="color:#ff0000;">3-I did not understand one thing about it.</span><br/><span style="color:#ff0000;">4-I did not understand a thing about of it.</span><br/><br/>In which case:<br/>a-there was one bit/one thing I did not understand, but I understood all the rest<br/>and in which case:<br/>b-I did not understand anything.<br/><br/>Gratefully,<br/>Navi.