Robert Jordan hears about El Sordo on the first night, from both Pilar and Anselmo. El Sordo seems like a top-quality guerilla.
For some unknown reason, El Sordo does not show up that night at Pablo's cave, so Pilar, Robert Jordan, and Maria decide to go see him the next day.
El Sordo meets Robert Jordan and Pilar (Maria in tow) at his hideout the next day and agrees to help Robert Jordan. He doesn't like the plan though, and suspects that the fascists already know about the Republican offensive.
As the two groups will not have enough horses between them to mount an escape, El Sordo agrees to go borrow a few from the fascists. They won't mind.
Some time later, El Sordo drops by Pablo's cave with a whiskey bottle for Robert Jordan (who's presumably off with Maria in the heathers). Then he's off to steal some horses.
It snows. It stops snowing. El Sordo and his fellow horse thieves leave tracks back to their hideout. El Sordo and his fellow horse thieves are screwed.
The next day, fascists attack El Sordo's cave. He and his band flee to a hilltop and fortify. They get to take quite a lot of fascists out (they stupidly charge up the hill) before being bombed by planes.