1. Números del 0 al 15 (Unique Names)

These numbers have unique names that must be memorized:

2. Números del 16 al 29 (One Single Word)

These are written as one single word. Note the spelling changes and written accents:

• 16: dieciséis (dieci + seis, needs accent)
• 20: veinte
• 22: veintidós (veinti + dos, needs accent)
• 26: veintiséis (veinti + seis, needs accent)

3. Números del 31 al 99 (Three Words)

From 31 onwards, numbers are written as three separate words using the conjunction “y” (and):

• 31: treinta y uno (thirty and one)
• 45: cuarenta y cinco (forty and five)
• 99: noventa y nueve (ninety and nine)

⚠️ Never write “treinta y uno” as one word, and never use “y” under 30 (e.g., NOT “diez y seis”).

4. El número 100

Use exactly cien for the number 100. Use ciento when followed by other numbers (e.g., 101 = ciento uno).

5. Apócope: 21 (veintiuno) & 31/41…

When 21, 31, 41, etc., are placed directly before a masculine noun, they shorten to veintiún or ending in -ún:

• veintiún libros (21 books)
• treinta y un pesos (31 pesos)