2.4 - Números 31 y adelante

With numbers that end in uno (31, 41, 51, etc.) uno becomes un before a masculine noun and una before a feminine noun

Ejemplos:

Hay treinta y un chicos.

Hay treinta y una chicas.

Cien is used before nouns & in counting

The words un, una, and uno are NEVER used before cien in Spanish

Use cientos to say hundreds

Spanish uses a period to indicate thousands and millions rather than a comma, as it is used in English.

You use ciento, not cien to count numbers over 100

Numbers 200 through 999 agree in gender with the nouns they modify (324 plumas --> trescientas veinticuatro plumas)

miles (the plural of mil), is used to express the idea of a lot, many or thousands (just like cientos is used to express hundreds)

To express a complex number (including years) string together all of its components (break it down by place value)