Hi Helen, We use "fell down" to refer to having failed in something OR collapsed. (Example: I fell down on my History paper.) (Example: I fell down (collapsed and dropped to the ground) when I was running and cut myself.) We use "fell over" to refer to something that toppled and fell down OR tripped over something. (Example: I fell over a rock (tripped over a rock) when I was running and cut myself.) (Example: The tree fell over and crushed the car.) Hope this helps. Cheers, Lance