Aplicaciones Y Reglas Para Los Artículos Indefinidos
by Katherine E. Rivas
1. 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.
1.1. Example
1.1.1. My brother is a doctor. George is a student.
2. Use a with names of jobs.
2.1. Example
2.1.1. John is a doctor. Mary is training to be an engineer
3. Use a with singular nouns after the words 'what' and 'such'.
3.1. Example
3.1.1. What a shame ! She's such a beautiful girl . What a lovely day !
4. Use a to refer to something for the first time
4.1. Example
4.1.1. Would you like a drink?
4.1.1.1. An elephant and a mouse fell in love.
5. Use a with nationalities and religions in the singular.
5.1. Example
5.1.1. John is an Englishman. Kate is a Catholic.