Spanish Numbers

Cardinal Numbers in Spanish

Cardinal numbers (cardinales) are the numbers used for counting; of course they look the same in Spanish as well as in English:

In Spanish uno becomes un when used in front of a masculine noun: un perro, veintiún perros. To refer to a feminine noun una is used: una mujer, treinta y una mujeres. Uno is used only when counting (uno, dos, tres) or for numerals refering to a masculine noun but does not precede that noun: ¿Cuántos hijos tienes? - Uno (or else we simply write) - un hijo

20 veinte
21 veintiuno/a
22 veintidós

21-29 are usually a single word composed as follows: omit the last e from veinte and add i instead then add the digit = veintiuno, the rest 30-99 can be formed up to three words: treinta y uno.

30 treinta
31 treinta y uno/a

So in short, uno in compound numbers loses the -o before masculine nouns, whether singular or plural: cuarenta y un días (forty-one days).
Dates (months and years) are cardinal numbers in Spanish, except for the first of the month: El 8 (ocho) de mayo de 1987 (mil novecientos ochenta y siete) (the eight of may, 1987). But: Hoy es el primero de enero (Today is January first).

Ordinal Numbers in Spanish

Ordinal numbers are used for positions, we say in English “first” “second” when referring to the position of someone/ something… same thing in Spanish:

After ten, cardinal numbers are generally used to indicate the ordinal numbers which makes it very easy for Spanish learners:
Luis trece (Lewis the Thirteenth)
el siglo veintiuno (the twenty first century)

Note that Spanish reverses the English usage of commas and periods in numbers: 1.250 kilómetros = 1,250 kilometers; 1,25 litros = 1.25 liters.