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1. Usually, the indefinite article 'A' is used in front of a noun (name) that begins with a consonant.

1.1. Example

1.1.1. A wide road

1.1.2. A large river

1.1.3. A car

1.1.4. A pet

2. The indefinite article ''an'' is used in front of noun (name) that begins with a consonant.

2.1. Example

2.1.1. An egg

2.1.2. An insect

2.1.3. An octopus

2.1.4. An airplan

2.1.5. An ugly hotel

2.1.6. An old book

3. The largest part of nouns and adjectives that begin with ''uni'' also take indefinite ''a'' article. When the ''u'' makes the same sound ''y'' in ''you''

3.1. Example

3.1.1. A university

3.1.2. A unit of measure

3.1.3. A unicorn

3.1.4. A uniform

4. Nouns beginning with ''eu'' or ''ur'' require the indefinite article ''a''.When ''eu'' or ''e'' make the same sound ''y''

4.1. Example

4.1.1. A euro

4.1.2. A European country

4.1.3. A urologist

4.1.4. A Eucalyptus tree

4.1.5. A uretic infection

5. Exceptions adjectives that begin with ''uni'' that are written ''an''

5.1. Example

5.1.1. An unimportant call

5.1.2. An unintended result

5.1.3. An unintelligent person

5.1.4. An unidentified flying objetc

6. When a noun begins with an aspirate ''H'', you have to use the indefinite article ''a''

6.1. Example

6.1.1. A hat

6.1.2. A house

6.1.3. A human

6.1.4. A helicopter

6.1.5. A hysterical joke

7. The unaspirate form is silent and do not add anything to the pronunciation of the noun. Phonetically, it's like the ''H'' does not exist. These examples and their variations represent all the words in English using the ''H'' unaspirate and written with ''an''

7.1. Example

7.1.1. An hour

7.1.2. An honest person

7.1.3. An hypothesis

7.1.4. An honor

7.1.5. An habitual offender

7.1.6. An herb ( in the pronunciation of England)

8. In Englis, the indefinite article is needed in front of the professions, see the examples with ''a''

8.1. Example

8.1.1. A lawyer

8.1.2. A doctor

8.1.3. A singer

9. Use ''a'' when ''o'' make the same sound of the ''w'' in ''won''

9.1. Example

9.1.1. A one-legged man

9.1.2. A one in the millon

10. Use ''an'' before the words that are written with consonant, but pronounced with a vowel sound

10.1. Example

10.1.1. An MBA

11. The indefinite article is used ''a'' for enumerations

11.1. Example

11.1.1. A hundred

11.1.2. A thousand

12. Used ''a'' as an auxiliary in a sentece when it comes a distributional vale

12.1. Example

12.1.1. Twice a month

13. Used ''a'' with ''what'' that is, as exclamacion

13.1. Example

13.1.1. What a nice day

14. Professions where you write ''an''

14.1. Example

14.1.1. An enginer

14.1.2. An actor

14.1.3. An architec