Numbers in Spanish (los números)

Un, Uno, Una, Unos, Unas?

Uno is male and una female.
>>Uno changes in un when used with a noun!
In plural (unos, unas) they mean 'some/few'

Una mesa
- a table
Un libro
- a book
Treinta y un libros
- 31 books
Una pila de libros
- a pile of books
Unas vacas
- some cows
Unos pollos
- some chickens

Primer, Primero, Primera

The ordinal numbers and fractions can occur both masculine (-o) and feminine (-a).
>>Primero and tercero lose their 'o' as adjective before a masculine noun.

El primer dia
- the first day
El tercer hombre
- the third man
El primero de mayo
- first of may
Un producto de la primera calidad
- a product of the first quality
El trigésimo cuarto libro de la lista
- the thirty-fourth book of the list
La trigésima cuarta ciudad del país
- the thirty-fourth city in the country

Place of the ordinal number

Ordinal numbers come before the noun.
>>However, if it is a royal person, a pope or a street, it is placed after the noun. In that case it's common, when it's under 10, to use the ordinal number, after 10 the cardinal number.

El quinto libro
- the fifth book
Carlos quinto
- Charles the Fifth
El decimocuarto día
- the fourteenth day
Luis Catorce
- Louis the Fourteenth
Calle cuarenta y dos
- 42 street
Papa Pío Doce
- Pope Pius the twelfth

Después de veintidos partidos, dos tercios de la liga, nuestro club de fútbol ocupa el decimotercer puesto
After twenty-two matches, two-thirds of the competition, our football club is thirteenth in the ranking.