is catherine still alive? Someone said that Elena ends up killing catherine. Is that true?

Katherine played a trick on Damon and Stefan, with her helper, they put the smouldering ash under the tree. If you've read the first book, you will know a lot was going wrong, and it was all Katherine's fault. Elena ends up killing Katherine.

+++SPOILER+++

And herself when she pushes Katherine in to the light.

yes. she is. katherine faked her own death. she wanted stefan and damon to get along. but that had the opposite affect.. and yes in the book elena kills katherine.

um... well in the tv show katherine is dead when Elena is alive.

yes she is. But elena kills her