Can you find any mistake??

I think the most impressed parts of this book are the Romantic period and the 19th century.
有2 part, 所以加s, 除非Romantic period and 19th century是同一個section, 那就用回part+is the, 但要加quotation marks "Romantic period and 19th century"
It introduces the development of music in Germany and France.
They introduce 或 It introduces (睇番到底計1 part 或2 parts)
Develop is a verb, should use the noun form - development
只有2個items 應用and作連接詞
It also introduces the variations of saxophone and piano.
And 和 also 意思有點重覆
This chapter provides a lot of informations on some of the most famous musicians, such as Frédéric François Chopin, Franz Liszt, Johannes Brahms and Charles Camille Saint-Saëns.
provides musician 意思有點不通順
最尾2個 items 應用 and作連接詞

On the other hand, guitar was very popular in that century, as well as operas from Italy.
可加一些轉接語句, 因已轉了topic
as well as 後不需再加 popular的形容, 因意思一樣
opera不需加 the, 因是泛指歌戲而非單指某幾套, 如要用 the的話可寫成 the Italian operas

Part iv) '-s' shouldn't be added in the word "information", because "information" is an uncountable noun.

The most impressed parts of this book were the romantic period and the 19th century. They introduced the development of music in Germany and France, as well as the variations of saxophone and piano. This chapter provided a lot of musician information; such as Frédéric François Chopin, Franz Liszt, Johannes Brahms, and Charles Camille Saint-Saëns. During this period of time, both guitar and opera from Italy were very popular too.

I think the most impressed parts of this book are about the Romantic period and the 19th century. The development of music in Germany and France is introduced. Different types of saxophone and piano are introduced. And this chapter provides description of many musicians, such as Frédéric François Chopin, Franz Liszt, Johannes Brahms and Charles Camille Saint-Saëns.

Guitar was very popular in that century and so did opera in Italy.

i changed a bit... >.<

i think the most impressed part of this book is the Romantic period and the 19th centur. It introduced the develop of music in Germany and France. And then it also intorduced the variation of saxphone and piano. This chapter talks about a lot of musicians, such as xxxxxx is Chopin, Franz Liszt, Johannes Brahms and Charles Camille xxxxxxxxx. Guitar is very popular in that century as well. Also the opera of Italy is very popular too.