Can you find any mistake???(1)

I think the most impressed part of this book is musical Romantic period and 19th century. (musician 應改為musical)
It introduces the development of music in Germany, France. (it 之後既introduce 有s, develop 係動詞, 應改做名詞development)
And then it also introduces the variation of Saxophone and piano.
This chapter introduces a lot of musicians. (provides 係供應, 你講本書介紹應該用introduce; a lot of 就係好多, 即plural, 所以musician 應有s)
Such as Frédéric François Chopin, Franz Liszt, Johannes Brahms, Charles Camille Saint-Saëns.
And guitar and piano are very popular in that century. (guitar and piano 兩樣野, 應用are)

I think that the most impressive part of this book is the music during the Romantic period and the 19th century.
- An impressive object/person makes you impressed.
- Musician period does not make sense. I guess that you mean the music during a period.
- Remember to add "the" in front of names of periods or centuries.

The book introduces the development of music in Germany and France.
- You can no longer use "it" to represent the book because the nearest singular noun is "the 19th century."
- To develop is a verb; development is a noun
- When you have only two items, you use the conjunction and.

The book introduces the development of music in Germany and France, and then it explains the variation between the saxophone and piano.
- It is better for you to join the two sentences into one. This provides a better flow of idea.
- I don't quite follow what you mean by "the variation of ..."

This chapter also presents a lot of musicians, such as Frédéric François Chopin, Franz Liszt, Johannes Brahms, and Charles Camille Saint-Saëns.
- If what you have written all came from the same chapter, I would suggest that you change the first sentence to "... the most impressived chapter ..." so that you can keep on referring to it.
- The chapter provides a list of musicians -- it cannot provide actual people, not to mention that the musicians listed are all dead :)

And the guitar and piano were very popular in that century.
- Since you were talking about something in the past, you should use the past tense.
- You need to add "the" in front of guitar (and it is assumed for the word piano) because you are singling out these music instruments and they are countable nouns.

I think the most impressing part of this book Romantic Period and 19th Century (is this the name of the book??) is the musicians. It introduces the development of music in Germany and France as well as the variation of saxophone and piano. This book also names many musician such as Drederic Francios Chopin, Franz Liszt, Johannes Brahms, and Charles Camille Saint-Saens. During that century, guitar and piano were very popular.

P.S. don't quite really understand the passage....!?

i think the most impressed part of this book is musician Romantic period and the 19th century. It introduces the develop of music in Germany and France. And then it also intorduces 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 guitar and piano is very popular in that century too.

think the most impressed part of this book is musician Romantic period and 19th century. It introduce the develop of music in Germany, France. And then it also introduces the variation of Saxophone and piano. This chapter provides a lot of musician. Such as Frédéric François Chopin, Franz Liszt, Johannes Brahms, Charles Camille Saint-Saëns.
And guitar and piano is(are) very popular in that century.