This question comes from Maria:

Why is "You swam very fast" correct, and "You swam very quickly" incorrect?

Fast indicates speed while quickly refers to the swiftness of the action itself.

Apple

Thanks to Apple for the suggested distinction between "fast" and "quickly."

While "swim fast" is far more common than "swim quickly," both adverbs are correct. We say that someone swims fast or that s/he is a fast swimmer. But "quickly" can also be used, and is found quite often when the action is one that is completed or that has an implied goal. That is, one can swim quickly from Point A to Point B, as in these Google examples:

"” "Marines (must) swim quickly to a drowning victim and still have enough energy to (bring the victim) back to safety," Lugo said.

"” This squid can't be defeated, so the only thing to do is swim quickly toward the surface.

"” Painted turtles have webbed feet so they can swim quickly away from an enemy or so they can swim quickly toward their prey.

Still, "quickly" is also used when there's no endpoint:

"” Whale could see the smoke from far away so he began to swim back to the island. He was not in a hurry so he did not swim very quickly.

"” Dolphins are very active. They swim very quickly and perform acrobatics. They like slapping the water with their chin and doing flipper slaps.

"” These [laterally compressed] fish are flattened from side to side and don't usually swim quickly, although there are exceptions.

"” Their front limbs have evolved into flippers, which allow seals to swim quickly and change directions with ease.

"” If a shark begins to get too interested in you by coming closer and closer, the best strategy is to leave the water - swim quickly but smoothly, ...

"” The webbed feet and streamlined body of a river otter allow it to swim quickly through the water.

BUT:

"” Some people don't really know why otters swim fast. They swim fast because they like to get away from other animals that are in the water

In sum, "fast" is the default adverb for "swim," but "quickly" is also acceptable, especially when it modifies an action that has an expressed or implied completion point.

Marilyn