If you want to say "first" in Spanish, there's a word for that — and it's nothing like uno, the word for "one." It's primero, the first of what are called the ordinal numbers.

Ordinal Numbers Function as Adjectives

The ordinal numbers might be thought of as the adjective form of the cardinal numbers, the numbers in the form they are most often used. Thus uno ("one") is a cardinal number, while primero ("first") is its ordinal form. The same goes for the cardinal dos (two) and the ordinal segundo (second).

In Spanish, the ordinal forms are used most commonly for the numbers 10 and under. They are:

First: primero

Second: segundo

Third: tercero

Fourth: cuarto

Fifth: quinto

Sixth: sexto

Seventh: séptimo, sétimo

Eighth: octavo

Ninth: noveno

Tenth: décimo

When used as an adjective, ordinal numbers must agree with the nouns they refer to in both number and gender: el segundo coche ("the second car," where coche is masculine), but la segunda vez ("the second time," where vez is feminine).

Note also that when primero and tercero precede a singular masculine noun, the final -o is dropped: el primer rey ("the first king"), el tercer trimestre ("the third trimester"). This change is known as apocopation.

For larger numbers, it is common to use simply the cardinal number, especially in speech. Thus el siglo veinte ("the 20th century") is more common than the cardinal form, el siglo vigésimo, and in writing the numerical (el siglo 20) or Roman (el siglo XX) form is often used. It is also common to word a sentence in such a way that the ordinal form is not used. So, for example, "cumple cuarenta y cinco años" (literally, she reaches 45 years) would be the most common way of saying it is someone's 45th birthday. In general, the ordinal numbers for 11th and above can be considered mostly a formal usage.

In any case, here are examples of larger ordinal numbers.

11th: undécimo

12th: duodécimo

13th: decimotercero

14th: decimocuarto

15th: decimoquinto

16th: decimosexto

17th: decimoséptimo

18th: decimoctavo

19th: decimonoveno

20th: vigésimo

21st: vigésimo primero

22nd: vigésimo segundo

23rd: vigésimo tercero

24th: vigésimo cuarto

30th: trigésimo

31st: trigésimo primero

32nd: trigésimo segundo

40th: cuadragésimo

50th: quincuagésimo

60th: sexagésimo

70th: septuagésimo

80th: octogésimo

90th: nonagésimo

100th: centésimo

200th: ducentésimo

300th: tricentésimo

400th: cuadringentésimo

500th: quingentésimo

600th: sexcentésimo

700th: septingentésimo

800th: octingésimo

900th: noningentésimo

1,000th: milésimo

2,000th: dosmilésimo

3,000th: tresmilésimo

4,000th: cuatromilésimo

1,000,000,000th: millonésimo

Ordinal numbers can also be written using a superscripted o or a depending on whether it is masculine or feminine, respectively. For example, the equivalent of "2nd" is 2o when referring to a masculine noun and 2a when referring to a feminine one. The lower-case superscripted o should not be confused with the symbol for degrees. Using ordinary lower-case letters (as in "2nd") is also possible when supercripts aren't available: 2o, 2a.

Mnemonic Device: Remembering the Ordinals

You may be able to help youself remember the ordinal forms by connecting them with English words you already know:

Primero is related to "primary."

Segundo is similar to "second."

Tercero is related to "tertiary."

A quarter, similar to cuarto, is a fourth of a whole.

Five children born together are quintuplets, using the same root word as quinto.

An octave, similar to octavo, has eight notes.

The decimal, similar to décimo, system is based on the number 10.

Sample Sentences Showing Use of Ordinal Numbers

El primer día fuimos amenazados por un grupo de manifestantes. (The first day we were threatened by a group of protesters.)

La estrella de muchas películas ha adoptado una segunda niña. (The star of many films has adopted a second girl.)

La Fórmula 1 contempla seriamente la opción de un tercer coche por equipo. (Formula 1 is seriously considering the option of a third team car.)

Hermine, la octava tormenta tropical de la temporada de huracanes en el Atlántico, se formó hoy. (Hermine, the eighth tropical storm of the hurricane season, formed in the Atlantic today.)

Puebla se ubica en el décimo lugar de las ciudades más caras del país. (Pueblo occupies 10th place of the country's most expensive cities.)

Esta es la lista de episodios pertenecientes a la decimosexta temporada. (This is the list of episodes from the 16th season.)

El empresario es el centésimo hombre más rico de Canadá. (The businessman is the 100th richest man in Canada.)

Eres la milésima persona que me dice que estoy muy guapo. (You are the 1,000th person to tell me that I am very handsome.)