just a comment...她是很漂亮...isn't actually wrong for all situations...I would use it to be more emphatic as in "she IS very pretty". 

A: 她很漂亮吗？

B: 还可以。。

A: 她是很漂亮，很多人都说。

This is so true. And those mistakes could be made even when the students become an advanced speaker if they didn't correct them at the beginning.

Thank you for these important reminders for improving our Chinese.

In his comments, andrew states that 是 can be used with adjectives for emphasis, eg.

她是很漂亮。

However, I learned that one should use the 是。。。的 construction for this purpose, e.g.

她是很漂亮的。

Are both correct?

That's why teachers should be more strict with the students at the very beginning. BTW, for the pronunciation difference between "zi" "ci" and "si", you will learn the pronunciation skills and tips from my videos on YouTube. <a href="https://www.youtube.com/watch?v=DDQbfDIG1mg">https://www.youtube.com/watch?v=DDQbfDIG1mg</a> ;