how to use the article "a" and "an"?

1. how to use "a"?

1.1. Use “a”, “and” only with singular nouns,

1.2. "a" is used when the nex woed starts whith a consonant

2. examples of the use of "a"

2.1. a union

2.2. a used napkin

2.3. ga chair

2.4. a dog

3. the article indifinido a becomes an if the following sustsntivo begins with a vowel

4. an orenge

5. how to use "an"

5.1. *Use "an" with words that start with a vowel sound

6. examples of the use of "an"

6.1. an apple

6.2. an elbow

6.3. an aardvark