3 The dog was playing _______ in the garden when it _______ to rain. (start)

В данном предложении необходимо использовать глагол start в форме Past Simple, так как действие произошло внезапно и однократно.

The dog was playing happily in the garden when it started to rain.

Ответ: The dog was playing happily in the garden when it started to rain.