I have a sentence:
It is a full-length movie about opera. If you are a novice in opera, this movie will make you love and understand opera, and, if you are so-called expert, then, by watching the movie, you will find out many things you didn't know about before.
Could you please correct mistakes? One of the big questions is commas.
Firstly, it should be "a" so-called expert ...
I think the placement of the commas is - strictly speaking - grammatically correct. However, the second sentence is beginning to look a bit complicated, so it would be best to re-write it and maybe turn it into two separate sentences which are less complex and would involve less use of commas. Something like ...
If you are a novice in opera, this movie will make you love it and understand it.
(I removed the second use of the word "opera" as it was being over-used at the start of your passage). Even if you are
a so-called expert, then watching the movie will still teach you many things that you didn't know before.
(For the second section, I made "watching the movie" part of the main sentence and used the word "teach" rather than "find out about", as it helps to simplify the sentence.)So, my finished suggestion would be ...
It is a full-length movie about opera. If you are a novice in opera, this movie will make you love it and understand it. Even if you are a so-called expert, then watching the movie will still teach you many things that you didn't know before.