1. Walking along the street, I met her.

Question

2. When I was walking along the street, I met her.
3. When I walked along the street, I met her.
* 4. Being walking along the street, I met her.
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Does #1 mean #2 or #3? Which one is close to #1? Is #4 ungrammatical? Can we find the explanation in a grammar book?

Answer 1

1, 2, and 3 mean the same thing.

4 is incorrect.
Having "being" and "walking" right next to each other is incorrect, plus it's wordy!

Read up on the use of participles as adverbs.