The difference between these two sentences is subtle, but it lies in emphasis and natural usage:
1. "I was arriving at the station, when it started raining."
- This sentence emphasizes the action of arriving. It sounds like the focus is more on your arrival and the timing of the rain is slightly secondary.
- It’s a bit less common to say it this way, as "arriving" is typically a shorter, more specific moment than an ongoing action.
2. "When I was arriving at the station, it started raining."
- This phrasing emphasizes the timing of the rain. It gives the impression that the rain starting is the main point of the sentence, while your arrival at the station sets the background context.
- This structure is more commonly used, as starting with “when” often highlights the importance of what happens next (the rain).
In general, the second sentence sounds more natural in English, especially because "arriving" is a brief action and is typically less continuous than other verbs that describe movement or duration.