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？

Question

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？

Answer

The tenses differ because:
1. In "was looking," the action of looking is continuous and ongoing at the time when another event (traversing the garden) happens.

2. In "arrived at," the action of arriving is completed before finding the people waiting.

The difference reflects the nature of each action—one ongoing, one completed.