Uno drops the "o" before masculine nouns in the singular, and the "o" changes to an "a" in front of feminine nouns. Compound numbers ending in "uno" behave the same way.
Ciento drops the "to" at the end before any noun. Multiples of "ciento" must agree in gender and number with the nouns they modify.
With numbers, decimal points and commas are reversed from the way that they appear in the United States. Every 3 places of larger numbers is divided by a decimal point, whereas whole numbers and decimals are divided by a comma.
EX. 1.000.000.000 = one million
EX. 1,5 = one and one-half
EX. un libro = one book
EX. una mesa = one table
EX. veinte y un libros = twenty-one books
EX. cien libros = 100 books
EX. doscientos libros = 200 books
EX. doscientas mesas = 200 tables
