In short

Italian numbers are written as one word and are invariable - only uno changes shape. The two spelling traps are the tens losing their vowel before uno and otto (ventuno, ventotto) and the accent on compounds of tre (ventitré). For the date use the cardinal (il 3 marzo) except on the 1st (il primo maggio), and for the clock say Sono le tre - where le is the plain definite article of an invisible ore.

The rule

Cardinal numbers

Step 1 - Learn 0-20 as vocabulary. There is no logic to find: zero, uno, due, tre, quattro, cinque, sei, sette, otto, nove, dieci, undici, dodici, tredici, quattordici, quindici, sedici, diciassette, diciotto, diciannove, venti. Notice the seam at 16/17: up to sedici the unit comes first (sedici = 6 + 10), from diciassette on the ten comes first (dieci + sette).

Step 2 - Build everything above 20 by gluing, in one word. Ten + unit, no space and no hyphen: ventidue, trentacinque, quarantasei, novantanove. This never stops: duecentoventicinque (225), quattromilacinquecento (4500).

Step 3 - Drop the ten’s final vowel before uno and otto. uno and otto are the only units starting with a vowel, and Italian refuses the collision. Every ten from venti to novanta elides:

• venti + uno = ventuno (not ventiuno), venti + otto = ventotto
• trenta + uno = trentuno, trenta + otto = trentotto
• quaranta + otto = quarantotto, ottanta + uno = ottantuno

All other units leave the ten intact: ventidue, ventitré, ventinove.

Step 4 - Put an accent on compounds of tre. Alone it is tre, with no accent. Glued onto a ten, the stress lands on that final syllable and must be written: ventitré, trentatré, quarantatré, cinquantatré. Forgetting the accent is a spelling error, not a shortcut.

Step 5 - Know which big numbers change. cento never changes (duecento, cento euro); mille takes the plural -mila, glued on (duemila, diecimila). Neither needs un in front. But milione and miliardo are true nouns: they take un, they pluralise, and they need di before what is counted - un milione di euro, due milioni di abitanti.

Step 6 - Agree only uno. Every number is invariable except uno, which behaves exactly like the indefinite article from step 1 when a noun follows: un libro, uno studente, una casa, un’amica. The same happens inside compounds: ventun anni, trentun giorni. Before anni, venti and trenta are usually clipped: Ho vent’anni.

Ordinal numbers

Step 7 - Learn the first ten, then use a machine. primo, secondo, terzo, quarto, quinto, sesto, settimo, ottavo, nono, decimo are irregular and must be memorised. From eleven upwards there is one rule: take the cardinal, cut its final vowel, add -esimo. undici -> undicesimo, venti -> ventesimo, cento -> centesimo, mille -> millesimo. Two spellings survive the cut: ventitré -> ventitreesimo (the e stays, the accent goes) and ventisei -> ventiseiesimo.

Step 8 - Treat ordinals as ordinary adjectives. Unlike cardinals they agree in gender and number, exactly like the -o adjectives of step 5: il primo giorno, la prima volta, i primi mesi, le prime settimane.

The date

Step 9 - il + cardinal + month. Months are lowercase and take no article: il 3 marzo, il 15 agosto, il 22 dicembre. That il is the definite article of an unspoken giorno, which is why it is masculine.

Step 10 - The 1st is the single exception. The first of the month uses the ordinal: il primo maggio, il primo gennaio. Every other day - all thirty of them - uses the plain cardinal. Never il terzo marzo.

Step 11 - Read years as one plain number. nel 2026 = nel duemilaventisei, where nel is not a chunk to memorise but in + il from step 7 doing its normal compulsory job. Months take a or in (a marzo, in marzo); the days - lunedì, martedì, mercoledì, giovedì, venerdì, sabato, domenica - go bare for one occasion (Arrivo lunedì) but take the article for a habit (il lunedì = on Mondays).

Telling the time

Step 12 - Ask with Che ore sono? (Che ora è? is equally correct and equally common.) The answer is elliptical: a feminine plural ore is understood but never said. Sono le tre is really Sono le (ore) tre - le is just the definite article, and the verb is plural to match it. It follows that:

• una is singular, so it takes the singular: È l’una.
• mezzogiorno and mezzanotte are masculine singular nouns with no article: È mezzogiorno.
• Everything else is plural: Sono le due, Sono le sette, Sono le ventidue.

Step 13 - Add minutes with e, subtract with meno. Past the hour: Sono le tre e dieci, le tre e un quarto, le tre e mezza. To the hour: Sono le quattro meno un quarto (3:45), le quattro meno cinque. mezza agrees with the hidden ora; mezzo is also heard and accepted.

Step 14 - Say “at” with a + the same article. No new forms here either - just the compulsory contraction of step 7 of the path, applied to the article you have just chosen: a + le = alle (alle tre), a + l’ = all’ (all’una). And where there is no article there is nothing to contract, so mezzogiorno stays bare: a mezzogiorno, a mezzanotte.

Forms & patterns

The cardinals worth knowing cold:

Where the ten keeps or loses its vowel:

Cardinal -> ordinal:

The clock, and the preposition that goes with it:

Examples

Common mistakes

• Ho ventiuno anni. -> Ho ventun anni. The ten elides before uno and otto: ventuno, ventotto. And before anni the number is clipped again.
• Sono ventitre studenti. -> Sono ventitré studenti. Compounds of tre are stressed on the last syllable, so the accent is compulsory - even though plain tre has none.
• Oggi è il terzo marzo. -> Oggi è il 3 marzo. Only the 1st takes an ordinal. Every other day is a bare cardinal after il.
• Sono le una. -> È l’una. The hidden noun is ora, singular: one hour takes a singular verb and the singular article.
• Ci vediamo a le tre. -> Ci vediamo alle tre. alle is not a time word - it is a + le, and that contraction is compulsory (step 7 of the path).

Tips

• The elision rule is smaller than it looks: uno and otto are the only units that begin with a vowel, so they are the only two that ever shorten the ten. Eight tens times two units - sixteen forms, one rule.
• cento and mille are bare (cento euro, mille euro), but milione and miliardo are nouns and need both un and di: un milione di euro. If you can put di after it, it is a noun, not a number.
• Time and date hide the same trick: an unspoken noun supplies the article. le (ore) tre is feminine plural; il (giorno) tre is masculine singular. That is the whole reason Sono le tre but il 3 marzo.
• Nothing about the clock is new grammar. Get the article right and step 7 of the path does the rest automatically: le tre -> alle tre, l’una -> all’una, mezzogiorno (no article) -> a mezzogiorno.