What's the difference between them? Example: A fallen tree blocked the path and we had to climb over it. Why can't we say "climb it" in this sentence.
"Climb a tree" means that you are going up into the tree. Generally this means you're sitting in the branches for some period of time.
"Climb over a tree" means you climb the tree and then come down the other side. I've only seen this used idiomatically if a tree has fallen into a path and you must climb it to keep traveling down your path.