Foxxi6, because you have mixed tenses, and Duo's sentence is supposed to be in the past tense.
If you want to say your sentence your way, it would be in the present tense, but you would have to say something like this:
"she falls and she hurts her knee",
but I can't think of anyone who would say it that way. The nearest sentence that would make any sort of sense in the present tense that I can think of, is:
"She is falling, and it looks like she is going to hurt her knee"
but realistically falls usually happen so quickly that they would have already happened before you could say the above sentence.
I hope that I have managed to explain what I mean adequately, and more importantly, correctly!