Hi,
In #1, it is an adjective (it could have been a red, or small car, for instance).
In #2, it is an adverb (he could also drive slowly)
See fast - English-Spanish Dictionary - WordReference.com for details.

Student or Learner
He has a fast car. He drives fast.
How would you explain the different ways ‘fast’ is being used in these two sentences in simple terms to a beginner level English speaker?
Hi,
In #1, it is an adjective (it could have been a red, or small car, for instance).
In #2, it is an adverb (he could also drive slowly)
See fast - English-Spanish Dictionary - WordReference.com for details.
Please be aware that I'm neither a native English speaker nor a teacher.
You could think of it as 2 words, both of which share the same spelling.
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