8. If it (not/start) to rain, we (walk) to the museum.

8. If it hadn't started to rain, we would have walked to the museum.

В условных предложениях третьего типа (Type 3) в придаточном предложении (if-clause) используется Past Perfect, а в главном предложении (main clause) используется would have + Past Participle.

Ответ: If it hadn't started to rain, we would have walked to the museum.