B. An Indefinite Article 'a' and 'an' is a determiner that introduces a noun phrase and implies to a general or non-specific thing, or to introduce new concept. (e.g. I bought a book; Jon went to a public school). 
1. to introduce a noun phrase and implies that a thing referred to is general or non-specific. (e.g. I am going to buy a book for you.)
2. used to introduce new concepts into a discourse. (e.g. A story is a promise letter for my future life.)
3. Generally, we use 'an' (not 'a') before a/e/i/o/u (e.g. a watch, an idea)

B1. Use Indefinite Article: 'a' or 'an'
1. with singular countable nouns to say what kind of thing or person is. (e.g. You're such a hard worker.)
2. use before singular countable noun when we talk about it the first time. (e.g. When we were on holiday, we stayed at a hotel. Sometimes we ate at the hotel and sometimes we went to a restaurant.)
3. to say what someone is or what job they do. (e.g. My brother is a teacher)
4. to show the person or thing is one of a group. (e.g. He is a student at London University), and to say something about all things of that kind (e.g. A whale is an animal.)
5. say that somebody has a long nose/ a nice face/ blue eyes (a part of body) (e.g. He has a nose for a good script.)
6. use as 'as per': once a week, twice a day, $10 a pound, etc.

B2. do not use the Indefinite Article with plural nouns and uncountable nouns. In the plural and uncountable noun, we use the noun only (e.g. They are such hard workers. Money is very important in our life essay, but it can’t be buy love and happiness. Water is your body's most important nutrient, is involved in every bodily function, and makes up 70- 75% of your total body weight.)