Use the word "fast" when describing speed or the movement of time. This word is similar to "quick."
He’s a fast runner. (He’s quick.)
She’s a fast worker. (She’s quick.)
This new computer is really fast.
The class went by fast. (It went by quickly.)
Time goes by fast when you’re having fun.
You said that too fast. Could you slow down, please?
You can use "fast" as an adjective or as an adverb. In the comparative form, use "faster," and for the superlative form use "fastest."
She’s fast. She’s a fast runner. She runs fast.
This is going faster than we expected.
He works the fastest of all the employees.
There are some other situations when the word "fast" is used:
He’s living in the fast lane. (He spends a lot of money and parties all the time.)
Live fast, die young. (If you try to have a good time all the time when you are young, drink a lot of alcohol or do drugs, you might die early.)
She played fast and loose with the facts. (She lied.)
These greens are fast. (In the game of gold, a fast green is the area near the cup. If the ball rolls quickly, we say the green is fast.)
You can also use "fast" as a verb or a noun when deciding not to eat food:
Muslims fast during Ramadan.
Fasting is a common practice in many religions.
Sadio isn’t going to lunch because she’s fasting.
Some people fast to lose weight.
He’s on a fast.
Cultural note: Do you like fast food? This is the kind of food that is prepared and consumed very quickly. McDonalds, Burger King, KFC, Pizza Hut, Subway and many others are popular fast food restaurants in the United States. The food isn’t very healthy, but it’s tasty and it’s cheap compared to food at a regular "sit-down" restaurant.