Short answers, pulled from the story.
Russia launched a military intervention in Syria on the 30th of September 2015 after receiving a formal request from the regime of Bashar al-Assad. The operation began with extensive air strikes across Syrian territory targeting strongholds held by opposition factions.
President Vladimir Putin stated that his chief objectives were maintaining the allied Ba'athist government in Damascus and capturing territories from American-backed Free Syrian militias. A broader geo-political objective was rolling back U.S. influence and blocking proposed pipelines between the Middle East and Europe to ensure Gazprom dominance over energy supplies.
The Syrian Observatory for Human Rights documented that Russian bombings directly killed more than 7,700 civilians by the end of April 2018, about a quarter of them children. Reports from September 2015 until February 2016 indicated at least 2,000 civilian deaths while other estimates placed the number at 1,700 including more than 200 children.
A treaty signed on the 26th of August 2015 permitted Russia to use Syria's Hmeimim airport free of charge indefinitely. This agreement ratified by Russia's parliament in October 2016 granted Russian personnel and their family members jurisdictional immunity and privileges as envisaged by the Vienna Convention on Diplomatic Relations.
On the 24th of November 2015 a Russian Sukhoi Su-24 strike aircraft was shot down by a Turkish Air Force F-16. The pilot was shot and killed by Syrian rebels while descending by parachute and a Russian marine died en route to a medical center after being injured during the rescue operation.