In Brueghel's painting, the fall of Icarus is happening in the back; no one is even noticing it. Brueghel's painting shows that how a tragedy may not even be noticed by those who are not directly affected by it.
Auden translates Brueghel's painting a little differently, however. To Auden, the fall of Icarus IS being noticed by the plowman and the sailor; Auden says that they are too busy to deal with the disaster and get on with their work. Auden says that Icarus falling is "not an important failure" to them.
Auden's interpretation of the painting is therefore much more negative than Brueghel's initial work.
We can't see underlining here, but here, I think this will be helpful:
"I read a novel" would be the intependent clause: it does not need the other one to be complete.
"until the guests arrived" is the subordinate clause: it needs the independent, main clause to be complete.
It can reveal how the writer has tried to engage the reader.
