The numbers in Spanish are straightforward with only a few spelling changes to remember.
The numbers from 0-30 are written as one word; from 31-99 they are written as three words separated by "y". For example:
| cero | diez | veinte |
treinta |
cuarenta |
| uno | once | veintiuno |
treinta
y uno |
cincuenta |
| dos | doce | veintidós |
treinta
y dos |
sesenta |
| tres | trece | veintitrés |
treinta
y tres |
setenta |
| cuatro | catorce | veinticuatro |
treinta
y cuatro |
ochenta |
| cinco | quince | veinticinco |
treinta
y cinco |
noventa |
| seis | dieciséis | veintiséis |
treinta
y seis |
cien |
| siete | diecisiete | veintisiete |
treinta
y siete |
ciento uno |
| ocho | dieciocho | veintiocho |
treinta
y ocho |
ciento dos. |
| nueve | diecinueve | veintinueve |
treinta
y nueve |
ciento tres... |
Notice that 16, 22, 23 & 26 require an accent mark.
The single-word numbers ending with one-syllable numbers (dos, tres, seis) need an accent mark.
This is because of the Rules of Pronunciation:
Rule #2 states that words ending in an N, S or vowel have the second to last syllable stressed: HA-blan, BUE-nos, FO-ca. Exceptions to this rule must wear a written accent mark.
Note: the accent mark also reflects the proper pronunciation. For example, it is more logical to say, veintiDOS after veinTIUno, than to change the pronunciation of veinte into *Vein-TI-dos.
Remember that any form of uno drops its "o" when used as an adjective:
Hay veintiún estudiantes. Tengo treinta y un años.