Complex sentence: Before 1906, the bridge used to be undone for the passage of vessels during daytime only, but since June of that year it started opening at night for all vessels except ocean steamers, which were required to pass through during daytime.
Split into simpler sentences: Before 1906, the bridge used to be undone for the passage of vessels during daytime only. Since June of that year it started opening at night for all vessels except ocean steamers, which were required to pass through during daytime.