Pasan y Pasan (They keep passing by) is a piece from Diez Piezas Fugaces (10 fleeting pieces). The name says it all, the piece is made up of a number of repeated patterns.
In this piece you need concentration on the right hand fingering in a finger-picking pattern. This becomes extra difficult in the variation of the arpeggio pattern in bars 17, 21, 25 and 29. This requires practicing slowly at first. Once you master the patterns, you can gradually increase the tempo.