1. ‎‎‎Looking through the window I saw him traverse the garden. （Change the sentence above↑ to below↓） When I was looking through the window I saw him traverse the garden. 2. Arriving at the airport she found a lot of people waiting for the security check. （Change the sentence above↑ to below↓） When she arrived at the airport,she found a lot of people were waiting for the security check. ↑The beginning of the two sentences is present participle, but after changing participles to verbs, the tense forms of the two verbs（was looking through、arrived at） are different. why？