Thank you so much for spending time checking my grammar mistakes <3 Is it ok if I have other questions: I thought that in this sentences [美しい桜や、寿司や刺身などいろいろな美味しい食べ物がある国だと思っていた], " 美しい桜", "寿司" and "刺身" are the examples of "いろいろな美味しい食べ物", which is incorrect for "美しい桜". And why in the first sentence I have to use "は" instead of "に"...

美しい桜　might be seen an example of these foods.
In that case, you can also say like this as an example to demarcate clearly.

美しい桜もあり、またお寿司やお刺身など色々なおいしい食べ物もある国だと思っていた。

「に」is impossible.
「の」is also possible.
The sentence can be like the following.
日本語の勉強を始める前の日本についてのイメージはシンブルだ。
「に」is ''at (that time)''
So that sentence you created before the correction means ''Before I studied Japanese, the image about Japan is simple.''
Oh, my goodness. Is this English sentence totally natural? lol (except the tense of ''is'' which should be ''was'')

So, there might be another problem.

始める前に、～をした。is possible.

For instance, 日本語の勉強を始める前に、フランス語の勉強をしていたことがある。
But you cannot say
始める前に、事実(～が・・・である）。
Before starting something, You acted something.
Only behavior can follow after that clause.

I am not sure my explanation is perfect. But I wrote what I was feeling.

Your explanation is really easy to understand <3 Pls adopt me as your student