Números 1-100

These cannot be derived, so learn them by heart. English turns regular at thirteen; Spanish keeps separate words up to quince (15).

16-19 start with dieci- and 21-29 with veinti-, followed by the unit, all in one word (from diez y ... and veinte y ...). There is no hyphen and no separate y.

Every ten is its own word. From 30 to 90 they all end in -nta, the way English tens end in -ty. 100 is plain cien, with no un in front.

From 31 on, join the ten, y and the unit as three separate words. Where English writes a hyphen (thirty-one), Spanish writes a little word. Only 21-29 are the exception: one word, no y.

Right before a noun, uno becomes un (masculine: un libro) or una (feminine: una casa). That also happens at the end of 21, 31, 41 and so on. In 21, un takes an accent (veintiún); in the written-out tens it does not (treinta y un).

Age, prices and house numbers – this is where you will actually use them. English says I am 25, but Spanish says age with tener (to have), not with ser.