My English teacher taught me about usage of "the" in any sentence.
There was one thing which he taught that you have to use "the" with any nationality which ends with "sh", "se", "ss" and "ch"

E.g.

> 
> She is the Polish.
> 
> 
> John is the Dutch.
> 
> 
> 

If there is nationality which does not end with above four letters then we can use "a" or "an"

> 
> He is an Indian.
> 
> 
> 

I can't find anything on Google to support this and I also do not feel that we need to use articles in above sentences.

Can someone help me with this.