Difference between です and をする in the following passage


I just came across this sentence while studying verb する

山田さんは英語の先生をしている - (As per the book it translated to " Yamada-san is an English
Teacher)

  * My question is why did not we use です instead of する and make the sentence into 山田さんは英語の先生です (which also has the same meaning)? 
  * Further, is there any difference in the nuance between the two sentences?




です is a statement that **A is B** , while している is a denotation of being so.

  * 山田さんは英語の先生をしている -- has been an English teacher. 
  * 山田さんは英語の先生です -- is an English teacher. 

On the first one, we surely understand she is doing now. The latter one is a
general phrase and also might 山田さん retired some time ago, but she is still a
teacher. In case of this, `先生をしていた` would fit perfectly.

