Question

1. An apple floats on water.
2. The apple floats on water.
3. Apples float on water.
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Are they all in generic use?
Are they all mean #4?
Does 'the apple' in #2 mean 'the specific apple'?

4. Any apple floats on water.

Answer 1

2 is not generic since it's talking about a specific apple

Answer 2

All are grammatically correct and generally in use, depending on the context.

And yes, in 2 it’s referring to a specific apple.