主句：there had come an unexpected crash of thunder.
从句：Once when my father had taken me for a ride in a sailboat
主句里的主语是an unexpected crash of thunder.谓语是come
如果不好理解主句主语,可以写成正常语序：An unexpected crash of thunder had come.
从句是由when引导的时间状语从句.从句里面主语是my father 谓语take 宾语me
方式状语for a ride 地点状语in a sailboat.