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

1.1. Example

1.1.1. An egg

1.1.2. An insect

1.1.3. An octopus

1.1.4. An airplan

1.1.5. An ugly hotel

1.1.6. An old book

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

2.1. Example

2.1.1. An unimportant call

2.1.2. An unintended result

2.1.3. An unintelligent person

2.1.4. An unidentified flying objetc

3. 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''

3.1. Example

3.1.1. An hour

3.1.2. An honest person

3.1.3. An hypothesis

3.1.4. An honor

3.1.5. An habitual offender

3.1.6. An herb ( in the pronunciation of England)

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

4.1. Example

4.1.1. An MBA

5. Professions where you write ''an''

5.1. Example

5.1.1. An enginer

5.1.2. An actor

5.1.3. An architec

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

6.1. Example

6.1.1. A wide road

6.1.2. A large river

6.1.3. A car

6.1.4. A pet

7. 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''

7.1. Example

7.1.1. A university

7.1.2. A unit of measure

7.1.3. A unicorn

7.1.4. A uniform

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

8.1. Example

8.1.1. A euro

8.1.2. A European country

8.1.3. A urologist

8.1.4. A Eucalyptus tree

8.1.5. A uretic infection

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

9.1. Example

9.1.1. A hat

9.1.2. A house

9.1.3. A human

9.1.4. A helicopter

9.1.5. A hysterical joke

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

10.1. Example

10.1.1. A lawyer

10.1.2. A doctor

10.1.3. A singer

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

11.1. Example

11.1.1. A one-legged man

11.1.2. A one in the millon

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

12.1. Example

12.1.1. A hundred

12.1.2. A thousand

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

13.1. Example

13.1.1. Twice a month

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

14.1. Example

14.1.1. What a nice day