Question

How did music differ before and after Bach?

Answer 1

Before Bach's time, music had very little base, so no proper foundations (tonality, proper scoring etc.). But after Bach's time, he had made the harpsicord's abilities really stand out with his 48 Preludes and Fugues. After Bach's works, more modulations, techniques and variance of tonal centre were established. So basically Bach was the pivoting point in music composition and partially performance.
Hope that helped :)
CO

Answer 2

Bach brought ABA and AB structures. In other words the music had more order afterBach.

Answer 3

J. S. Bach didn't actually have a huge impact on the music written immediately after his death. It took a while for his music to be appreciated for what it was. Music before Bach often used ancient greek modes and was more concerned with horizontal progression of pitch (voice leading) than vertical alignment of pitch (harmony). In other words, before Bach, chords and harmonic progression were simply a consequence of several horizontal lines following the rules of counterpoint and were not really considered important in their own right (Look up Gradus ad Parnassum by Joseph Fux [ http://www.opus28.co.uk/Fux_Gradus.pdf ] for an idea of what these rules were). Bach aided in the establishment of the harmonic aspect of music while still sticking with the rules of counterpoint that were in place before him. He was basically the last of the great Baroque composers. Rather than completely revolutionising music, he developed the stylistic ideas of his predecessors as far as they could go.

After Bach's death, the new generation of composers grew tired of the complexity of heavily contrapuntal late Baroque music and sought a simpler style of music. This simpler style traded counterpoint for light and decorative melodies accompanied by clear chord progressions and simpler versions of the binary and ternary forms of the Baroque suites and sonatas, leading eventually to sonata form. The most influential composer in this style was actually J. S. Bach's son, C. P. E. Bach. I recommend giving his music a listen. He is a criminally underrated composer today but he was well celebrated in his own time and a major influence on Haydn and Mozart. If you listen to C. P .E. Bach's music, you will hear what I am talking about. On the one hand, it sounds like a Baroque concerto grosso; on the other, it sounds almost like it could be written by Haydn or Mozart. As for J. S. Bach's music, it had grown in stature by the end of the 18th century and was influential on Beethoven and has been on pretty much every composer who has followed.