The literal meaning of "leaves fall" is that they drop down to the ground from the tree. The literal meaning of "leaves float" is that they sit on top of the water, like a boat. But pretend you are standing in the forest and it is autumn. The leaves are turning yellow and red and orange. The wind comes along and blows the leaves off of the tree and carries them along in the air without letting them fall to the ground. How can you describe this? You could say that the "yellow leaves float on the cool wind", because it LOOKS like they are floating on a river of wind.

'leaves float' would refer to them floating on the surface of some water. You could say that leaves 'float to the ground', or possibly 'float down', to refer to them falling slowly, but this is a little poetic, and you can't use 'float' on its own to refer to that.

So in this blank fall and float are both right?

From my school exercise book，there's a sentence'I could see the yellow leaves ____ on the cool wind.'I filled in the blank with the word 'fall'.But the answer is float.

Although there is a little bit of overlap in certain phrases, 'fall' and 'float' are verbs with very different meanings. To 'fall' is to drop because of gravity. A pencil will 'fall' if you let go of it, and it will stop falling when it reaches the ground. To 'float' is to resist falling. Leaves 'floating' are falling slowly. A leaf can fall to the ground - this is acceptable use of the verb. A leaf can ''float' to the ground - this is a more descriptive way to say that a leaf is falling slowly.