particles - Differences between は and が

I am reading this article, however, I am not sure if I've already grasped that article correctly or not, so I really need someone to confirm whether my understanding is correct or not.

First question

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  Suppose I want to tell everyone a pen belongs to me by saying "これは私のペンです." so just in case I lost it somewhere and someone found it, he will know that pen is mine.

  
  


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  Suppose there are 2 pens on the floor and then I pick mine and say "これがわたしのペンです."

  
  

Did I grasp it correctly?

Second question

In that linked article there is an example "日本語が上手ですね！" Can I change it using は as in "あなたの日本語は上手ですね！" without changing the meaning?

Answer

The は can be used for comparing.
For example, saying あなたの日本語は上手ですね！ means that your JAPANESE is good, unlike all your other languages. So the meaning is quite different.

To push it to the extreme, a girl might slap you if you say あなたの目はきれいです。Because in a way it implies that the rest of her body is not.