Fast, quick or quickly?

Fast and quick mean moving with great speed. Fast is both an adjective and an adverb. Quick is an adjective and the adverb form is quickly.

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It was a fast train.

We need to have a quick chat before the meeting.

Fast and quick are adjectives.

You walk very fast!

Not: … very fastly.

We should do it as quickly as possible.

Fast and quickly are adverbs.

We usually use quick to refer to something happening in a short time, or a shorter than expected time:

We stopped for a quick snack.

Not: … a fast snack.

I just need a quick answer.

Not: … a fast answer.

Fast refers to things that happen or go at high speed, e.g. a train, a person running:

She loves fast cars.

Not: She loves quick cars.

Note the common expression fast food to refer to food such as hamburgers, which are cooked and served in a very short time in restaurants:

You shouldn’t eat so much fast food. It’s not good for you.