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.
I believe you were either mistaught, or mislearned the rule.
*She is the Polish.
*John is the Dutch.
These sentences are both incorrect. The correct forms are as follows:
She is Polish.
John is Dutch.
The situation where you use "the" is when talking about general groups of people:
The Dutch live in the Netherlands.
And this is only in particular circumstances. What your rule seems to be getting at is that, with a word like "Dutch", you need the definite article in this context ("Dutch live in the Netherlands" doesn't sound natural). However, with a word like "Indians" that takes the plural suffix -(e)s, the definite article is not required for general statements:
Indians live in India.
It is possible to use "the Indians"; it just has a slightly different meaning from "Indians" without a definite article. (The version with a definite article has a slightly more absolute or collective sense, while the version without the definite article gives more of a sense of a generalization or stereotype that is thought to be usually true, but not necessarily true for every last Indian.)
In general, nationality words can be divided into several types.
Adjective and noun are the same form. For this type, the only way to talk about groups is by using the -(e)s plural, with or without a definite article "the". Examples: (the) Indians, (the) Americans, (the) Israelis, (the) Arabs, (the) Greeks. Common suffixes for words in this class: -(i)an and -i.
Noun with no suffix, derived adjective that ends in the suffix -ish. For this type, groups are usually talked about using the -(e)s plural of the noun, but (the) __ish may occasionally be heard or seen. Some might consider "the __ish" an error for words in this class, however. Examples:
Adjective with the suffix "-ish", "-sh" or "ch"; noun formed from the adjective plus "-man" or "-woman". For this type, groups are talked about using "the __ish/sh/ch". Examples:
Adjectives with the suffix -ese, and Swiss; these have no standard noun form. For this type, groups are talked about using "the __ese". Examples:
...And there are even more patterns that I've probably forgot, like "Spaniard" (n.), "Spanish" (adj.), "the Spanish".
Also, see "Why can we say 'an American' but not 'a British'?" You might occasionally see an "-ese" word used as a noun (like "a Chinese"), although this seems awkward to native speakers.
Your teacher is incorrect, and you do not necessarily need an article to describe where someone is from. It depends on whether you are using an adjective (the nationality) or a noun (a word to describe the person) after the "to be" verb.
For example, it is is correct to say both "I am Australian" and "I am an Australian". The first is using Australian as an adjective whereas the second is using it as a noun.
It is also correct to say "I am the Australian". However, the definite article means that in saying this, you are referring to a specific person (who has been referred to before).
However, not all countries have the same word for the noun and the adjective. In the case of people from Poland, the noun is "Pole" whereas the adjective is "Polish".
Fred is from Poland. He is a Pole. He is Polish.
He is the Pole I was talking about earlier.
This site breaks it down in a bit more detail and has a comprehensive list of the adjectives/nouns/plural nouns for different countries.
