second-person singular simple present form of fast
"You fast when you do not eat food for a period of time to purify your body or spirit."
In plain English: Fastest is not actually used as a verb; it is an adjective that describes something moving more quickly than anything else.
"The fastest mouse in the room dashed across the table to catch the cheese."
superlative form of fast: most fast
"The cheetah is the fastest animal on Earth, capable of reaching speeds up to 70 miles per hour."
In plain English: Fastest means moving more quickly than anything else around you at that moment.
"The fastest runner in our group won the race easily."
Usage: Use the irregular superlative "fastest" instead of adding "-er" or "-est," as it is not formed by standard suffixation. This word functions identically whether modifying a noun like "the fastest car" or describing an action such as "running fastest."
superlative form of fast (adverb): most fast
"She ran the fastest to catch the bus before it left."
In plain English: Fastest is used to describe something that moves more quickly than anything else around it at this moment.
"She runs faster than anyone else in her class, making her the fastest runner on the track team today."