10. While we (walk) home, it (start) to rain.

10. While we (walk) home, it (start) to rain.

В данном предложении необходимо употребить Past Continuous для действия, которое происходило в определенный момент в прошлом (while we were walking), и Past Simple для действия, которое прервало первое (it started).

While we were walking home, it started to rain.

Ответ: While we were walking home, it started to rain.