One is un before a

1
Regla 1
One is un before a masculine noun un libro
One is una before a feminine noun una casa
The Spanish words for 21, 31, 41, etc. also obey
this rule veinte y un (veintiún
cuadernos) veinte y una (veintiúna plumas)
2
Regla 2
The numbers 16 to 19 and 21 to 29 may be written
as three words or as 1 word

diez y siete or diecisiete
veinte y nueve or veintinueve

páginas
chicos
The following words take accent marks
dieciséis, veintiún, veintidós,veintitrés,
veintiséis
3
Regla 3
Except for 1, 21, 31, etc., Spanish number-words
do not change in gender
cuatra casas
cuarto casas
4
Regla 4
cien vs. ciento
In numbers 101-199 ciento is used
cien vasos 100 glasses cien sillas 100 chairs
ciento un chicos 101 boys ciento una chicas
101 girls
ciento cuarenta y siete dólares 147 dollars
5
Regla 5
mil 1000
1000 mil 1009 mil nueve 1034 mil treinta y
cuatro 1114 mil ciento catorce 1575 mil
quinientos setenta y cinco 1991 mil novecientos
noventa y uno
Cuándo naciste? ______________________________
6
Regla 6
The conjunction y occurs only between the tens
and ones digits
cuarenta y cinco (45) but tres mil siete (3007)
7
Regla 7
The numbers 200 to 900 end in os if followed by
a masculine noun, and in as if followed by a
feminine noun. Note that this gender agreement
occurs even when other number-words come
between the hundreds and the noun.
Trescientos hombres 300 men Trescientas
mujeres 300 women Mil novecientos once
edificios 1911 buildings Mil novecientas once
páginas 1900 pages