1. She can run as fast as him.

Question

2. She can run as fast as he.
3. She can run as fast as he can.
4. She can run as fast as he can run.
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Are they all grammatical? Can we use all of them?

Answer 1

1 is incorrect.

2 and 3 are the most often used.

4 is correct, but wordy and not used much.

Answer 2

5. She can run as fast as he can run fast.
6. She can run fast. He can run fast.
=> She can run as fast as he can run fast.

[Thank you for your help. What about these ones?]

Answer 3

Those are too wordy/repetitive. Here's the best way to say this:

She can run as fast as he can.