Ordinal numbers indicate the order or position of something (in English they are “first, second, third...fourteenth, fifteenth, etc.” Italian ordinal numbers are irregular from first to tenth, and they always function like an adjective: they must agree in gender and number with the noun they modify, e.g.,
il primo negozio = the first store
la prima borsa = the first bag
i primi soldati = the first soldiers
le prime luci = the first lights
Following are the ordinal numbers, “first” through “tenth” in their natural state (ending in –o)
For every ordinal number beyond “tenth,” you simply elimate the final vowel from the cardinal number and add –esimo/a/i/e.
la quarantesima maratona (quaranta + esima)
the fortieth marathon
il quindicesimo piano (quindici + esimo)
the fifteenth floor
il venticinquesimo compleanno (venticinque + esimo)
the twenty-fifth birthday
If your ordinal number ends in a “three” like twenty-third, ventitreesimo, then you double the E:
trentatreesimo = thirty-third
Fractions are expressed in Italian with cardinal numbers first, and then ordinal numbers (this is what we do in English, also):
due terzi = two-thirds
quattro quinti = four-fifths
However a fraction with “one” in the beginning is written with un:
un terzo = one third
un quinto = one fifth