Hi Alexey - "fast" and "rapid" are very similar! There is a small difference though. "Rapid" describes movement towards a specific conclusion/direction. For example, in English we have the phrase "rapid transit" in a city which is usually a train or bus or metro that moves in a straight line with the purpose of getting to a destination without many stops. "Fast" does not have the strict association with direction and can be used to describe something like "Do your homework fast so you can join our pizza party tonight!" When you do your homework fast there is no movement towards something in space, it's just that the time you take to complete the homework is short! The origins of the two words are different too - "Rapid" comes from Latin/French. "Fast" is of old English/Germanic origins :)

Fast and rapid are synonyms. But it depends on how they are used. We would talk about a car being fast, not rapid. I would also say that rapid means 'very fast'.

The other answerers gave you fantastic info. I just want to add a connotative perspective which may be less helpful for some learners and more helpful for others. When I read "rapid", it adds a sense of repetitive motion. When I read "fast", it emphasizes speed. For example: 1. The runner's rapid pace around the track won them the race. 2. The runner's fast pace around the track won them the race. In the first example, there is an emphasis on the steady repeated motions of the runner's stride. I might picture this in my head as a tortoise and the hare scenario, where the racer isn't necessarily faster than the others but their dedication and strong movements help them win. In the second example, the emphasis is on the speed of the runner and I think of them as being swifter. I might imagine this race to have a larger margin between the 1st and 2nd place runners. The fast runner has rapid steps. The student writes fast. Their pencil waves rapidly across the page. The fast car spins its wheels rapidly. After a fast run, the dog's panting is rapid.

"Fast" refers to speed. "Rapid" belongs to a family of words related to the idea of snatching, carrying off, seizing, or grabbing: "raptor" "rapacious" "rapture" "rapt" "ravish" "ravage" "rapine" For example a "raptor" is a bird, like an eagle, that snatches its prey and carries it away. A "rapid train" does something similar for you. It snatches you and carries you away to another place. "Fast" is both an adjective and an adverb but "rapid" cannot be an adverb. Instead, you would have to say "rapidly".

Here are the main differences between "fast" and "rapid": Speed: Both words describe speed, but "fast" is more general, while "rapid" often implies a high speed over a short period. Usage: Fast: Commonly used for people, vehicles, actions, or anything that moves quickly. Example: "a fast car," "a fast runner." Rapid: Often used for changes, movements, or developments. Example: "rapid growth," "rapid response." Connotation: Fast: Can imply quickness but not necessarily suddenness. Rapid: Suggests suddenness or quick development. In Summary: Fast is broader and more versatile. Rapid is more specific, often suggesting a swift and sudden action.