Although Spanish numbers can be memorized in a straightforward way, their use can be confusing for persons new to Spanish. Numbers made up of more than one part are often formed differently than they are in English, and some Spanish numbers change according to the gender of the nouns they apply to.
Following are the basic Spanish numbers and patterns in which they are formed. Those that are in italics are forms that change according to gender, while the non-italic forms are fixed.
Shortening uno and ciento: Uno and numbers ending in -uno are shortened to un when they immediately precede a masculine noun. When standing alone (that is, being 100 exactly) ciento is shortened to cien before preceding a noun of either gender; the longer form is used within longer numbers (except when preceding mil).
Gender: As indicated above, the italicized portions of numbers vary according to gender. When a number ends in -uno ("one"), the form -un is used before masculine nouns, and -una before feminine nouns. The uno form is used only in counting. Accent marks are used where needed to maintain the correct pronunciation. The hundreds portions of numbers change in gender even when other parts of the number intervene before the noun.
Punctuation: In most of the Spanish-speaking world, periods and commas within numbers are reversed from what they are in U.S. English. Thus in Spain 1.234,567 would be the way of writing mil doscientos treinta y cuatro coma quinientos sesenta y siete, or what would be written in the United States as 1,234.567. In Mexico, Central America and Puerto Rico, numbers usually are punctuated as they are in the United States.