grammar.numbers.cardinal
Cardinal numbers: uno to millones
16–29 fuse (dieciséis, veintiuno); 31+ use y (treinta y dos); cientos agree (doscientas personas); mil never takes un; millón takes de before nouns.
grammarA1✓ Reviewedv0.1.0
Explanation
Cardinal numbers have a few mechanical rules. The teens and twenties fuse into one word (dieciséis, diecisiete, veintiuno, veintidós), but from 31 up they're written with y between tens and units: treinta y dos, cincuenta y siete. The y appears only there — never *ciento y treinta.
Uno agrees and shortens: it becomes un before a masculine noun (un café, veintiún años) and una before a feminine one (una página, veintiuna casas). Cien is used alone and before bigger numbers (cien personas, cien mil), but it's ciento in 101–199 (ciento cinco); the hundreds 200–900 agree in gender: doscientas personas, cuatrocientas casas.
Mil is invariable and never takes un: mil dólares, dos mil, cien mil. Millón, by contrast, is a noun and takes de before what it counts: un millón de habitantes, dos millones de pesos.
Examples
That's five hundred twenty-one tickets.
Region: global
The city has two million inhabitants.
Region: global
We sold five hundred twenty-one tickets, almost a million bolivianos.