Numbers in Spanish -LOS NÚMEROS EN ESPAÑOL- aren’t difficult to learn, you just have to pay attention to some patterns.
Like in English we can organice them into two main groups: cardinal (the one that we are going to see today) and ordinal numbers.
Spanish numbers from 0 to 99
1. Uno and numbers ending in uno, such as 21 or 91, change to agree with the following noun. They drop the final –o before a masculine noun and have the feminine form una: Tiene cuarenta y un años (He is forty-nine years old). El libro cuesta veintiuna libras (the book costs twenty-one pounds).
2. From 11 to 15 all end in –ce: once, doce, trece, catorce, quince.
3. From 16 to 19 have a different pattern, starting with diez (ten). They were originally diez y seis (ten and six), diez y siete (ten and seven), but become contracted into one word: dieciséis, diecisiete, dieciocho, diecinueve.
4. From 21 to 29 are also contracted numbers, with the final “e” of veinte (twenty) changing to “i”: veintiuno …. veintinueve.
5. Notice that the following numbers have a written accent: dieciséis (16), veintidós (22), veintitrés (23),veintiséis (26).
6. From 30 to 90 all end in –nta and have y (and) between the tens and units: treinta y cinco (35), cincuenta y ocho (58), noventa y nueve (99).
Spanish numbers from 100 to 999
7. 100 is just written CIEN.
8. Ciento, not cien, is used before a number after 100: ciento doce (112), ciento ochenta y cinco (185)…
9. Ciento adds -s in the plural, and doscientos, trescientos… novecientos, agree with their noun: quinientos gramos (5oo grams), ochocientas libras (800 pounds)…
10. There is no “y” and between hundreds and tens: trescientos veinte (320), novecientos doce (912), but novecientos sesenta y dos (962)…
Spanish numbers from 1000
11. Mil (1.000) never changes and never has “un” a before it: dos mil (2.000), cinco mil trescinetos veinticinco (5.325)…
12. Un millón (1.000.000) y un billón (1.000.000.000.000) have regular plurals: millones, billones. They are followed by “de” when used alone before a noun:
Un millón de euros
Cinco millones de libras esterlinas.
Un billón de dólares.