In English, the two indefinite articles are a and an. Like other articles, indefinite articles are invariable. You use one or the other, depending on the first letter of the word following the article, for pronunciation reasons. Use a when the next word starts with a consonant, or before words starting in u andeu when they sound like you. Use an when the next word starts with a vowel (a,e,i,o,u) or with a mute h.

The indefinite article is used to refer to something for the first time or to refer to a particular member of a group or class. Some use cases and examples are given below.

Use a to refer to something for the first time.

NAMING MEMBERS OF A GROUP

Use a with names of jobs.

Use a with nationalities and religions in the singular.

Use a with the names of days of the week when not referring to any particular day.

Use a to refer to an example of something.

Use a with singular nouns after the words 'what' and 'such'.

Use a meaning 'one', referring to a single object or person, or a single unit of measure. In these sentences using "one" instead of the indefinite article is grammatically correct. It will add emphasis to the number, and contrast with other numbers.