Regarding the ongoing Syrian Civil War, who are the important actors and what are their preferences over possible outcomes? What factors help or hinder states in reaching a cooperative solution in this war? And has any international institution been helpful in trying to resolve this civil war?

There are actually three key groups at play in the Syrian Civil War, which has now been underway for over six years. These groups are:
Islamic State, or Daesh, a terrorist organization who are opposed to the current president, Bashar Al-Assad.
Bashar Al-Assad as a representative of the legal government of Syria, and his supporters. Importantly, he is backed by the national army.
"Rebel" fighters, who are not allied to Islamic State but oppose the government.
Unrest began in Syria in 2011 because it was felt that the government was not sufficiently democratic: protests led to the murder of five people by the government, which drove the rest of the country to action. While the protesters originally only wanted greater transparency in government, they soon came to call for the removal of Al-Assad and his government from power. Al-Assad continues to refuse. So, the government's goal is to reassert its power over the country. The rebels' goal is to dissolve the current government. Meanwhile, Islamic State also wants the government to be dissolved, but for its own reasons: it has been taking over lands in Iraq and expanding into Syria. Its aim is to spread its hateful extremist message as far and widely as possible. In actual fact, the civil war in Syria is three-sided, in that both the government and the rebel fighters are attempting to beat IS, which is not "on the side of" the rebels, as they are opposed to IS as well as to Al-Assad. They want democracy, which IS most certainly does not represent.
In attempting to reach a solution, this is certainly presenting a problem. Civil wars are always more complicated when there are more than two groups fighting; it is suggested that there are thousands of different factions with differing goals fighting against the Syrian government. Meanwhile, both the US and the UK have intervened with air-strikes against IS, but these have also had the side-effect of destroying many civilians' houses. However, an agreement between the US and Russia in 2013, which resulted in the destruction of chemical weapons in Syria, has lessened some tensions.
Many European countries have taken in significant numbers of refugees fleeing the war in Syria, but this has been hotly contested in some areas, with the US and UK governments, in particular, anxious about the possible spread of Muslim extremists.
https://www.bbc.co.uk/newsround/16979186