1. Walking on the street, I found a stray dog.

2. While I was walking on the street, I found a stray dog.

3. When I was walking on the street, I found a stray dog.

4. While I walked on the street, I found a stray dog.

5. When I walked on the street, I found a stray dog.

(Among the four sentences, which one is similar to Sentence 1?)

All sentences 2-5 mean the same as sentence 1.

=)

Sentence 3 is similar to Sentence 1. Both sentences convey the same meaning, indicating that the action of finding a stray dog happened at the same time as the action of walking on the street. The conjunction "when" in Sentence 3 shows the simultaneity of the two actions, just like in Sentence 1 where there is no explicit conjunction, but the timing of the actions is still implied.