1. The indefinite article an
1.1. Applications
1.1.1. It is used in front of a noun that begins with a vowel.
1.2. Rules
1.2.1. When the noun that accompanies the article begins with a vowel (or a consonant with a vowel sound like "h" that is not pronounced), it is used in the article "an". (an apple = an apple / an office = an office / an hour = an hour)
1.3. Examples
1.3.1. She saw an elephant.
1.3.2. They have an idea
1.3.3. The boy always eats an apple in the mornings.
1.3.4. You need an umbrella because it’s raining a lot.
1.3.5. An ant was walking on my pillow.
2. The indefinite article a
2.1. Applications
2.1.1. Is used in front of a noun (noun) that begins with a consonant.
2.2. Rules
2.2.1. When the noun that accompanies the article begins with a consonant (or a vowel with a sound of consonant), the article "a" is used. (a car = a car / a book = a book / a useful thing = a useful thing).
2.3. Examples
2.3.1. Do you have a pen?
2.3.2. They have a car.
2.3.3. The dog is playing with a ball
2.3.4. I bought a car
2.3.5. I am reading a book.