The largest and most regular class of Romanian adjectives has four distinct forms — one each for masculine singular, feminine singular, masculine plural, and feminine plural. The model is bun / bună / buni / bune ("good"). If you have already learned how Romanian nouns form their plurals, you have already learned most of this class for free: four-form adjectives undergo the same vowel and consonant alternations as nouns. That is the big payoff of this page — the two systems are one system.
The basic four-form pattern
Take a masculine-singular adjective ending in a consonant. The other three forms are built by adding endings to the same stem:
- masculine singular — bare stem: bun
- feminine singular — add -ă: bună
- masculine plural — add -i: buni
- feminine plural — add -e: bune
| Singular | Plural | |
|---|---|---|
| Masculine | bun | buni |
| Feminine | bună | bune |
un câine bun
a good dog (masculine singular)
o pisică bună
a good cat (feminine singular)
câini buni
good dogs (masculine plural)
pisici bune
good cats (feminine plural)
Dozens of everyday adjectives take these same four endings on a plain consonant stem: înalt ("tall"), deștept ("clever"), sărat ("salty"), gras ("fat"), slab ("thin/weak"), bogat ("rich"), urât ("ugly"). Learn the four endings once and you can inflect them all. (Some four-form adjectives add a vowel twist on top — frumos and sănătos shift their o to oa in the feminine — covered in the next section.)
The o → oa alternation (frumos → frumoasă)
Romanian has a famous sound rule: a stressed o in the masculine becomes the diphthong oa when followed by an -ă or -e ending. This is exactly the alternation you see in nouns (sat / sate stays, but poartă / porți and floare / flori show the same oa↔o seesaw). In four-form adjectives it produces the frumos pattern:
| Singular | Plural | |
|---|---|---|
| Masculine | frumos | frumoși |
| Feminine | frumoasă | frumoase |
The masculine singular keeps the plain o (frumos, frumoși), but the feminine forms — which end in -ă and -e — turn it into oa (frumoasă, frumoase). The same happens to gros ("thick") → groasă, sănătos ("healthy") → sănătoasă, călduros ("warm") → călduroasă, gustos ("tasty") → gustoasă.
un pulover gros și o haină groasă
a thick sweater and a thick coat (gros → groasă: the o becomes oa)
o supă gustoasă, niște prăjituri gustoase
a tasty soup, some tasty cakes
A fost o vară foarte călduroasă anul ăsta.
It was a very warm summer this year.
The trigger is the ending vowel, not the gender as such — but since the -ă and -e endings are precisely the feminine ones, in practice "o → oa" happens in the feminine. This is the single most common alternation in the four-form class, and English speakers regularly miss it (writing frumosă instead of frumoasă).
Consonant changes before -i (masculine plural)
The masculine plural ending -i is not just tacked on; it can soften or change the preceding consonant, just as it does in masculine noun plurals (pom / pomi, brad / brazi). The most important changes:
| Stem ends in | Becomes before -i | Example (m.sg → m.pl) |
|---|---|---|
| -t | -ți | înalt → înalți |
| -d | -zi | crud → cruzi |
| -s | -și | frumos → frumoși |
| -st | -ști | prost → proști |
| -sc | -ști | românesc → românești |
oameni înalți
tall people (înalt → înalți: t softens to ți)
niște cartofi cruzi
some raw/uncooked potatoes (crud → cruzi: d → zi)
copii proști
foolish children (prost → proști: st → ști)
obiceiuri românești
Romanian customs (românesc → românești: sc → ști)
If you can already produce the masculine noun plurals brazi, pomi, români, you already know how to produce cruzi, înalți, românești. The rule is identical — it is the same -i doing the same work.
When some forms happen to look alike
A four-form adjective is defined by its agreement behavior, not by always having four visibly different spellings. With some stems, two of the four forms come out identical by accident. The classic case is mic ("small"):
| Singular | Plural | |
|---|---|---|
| Masculine | mic | mici |
| Feminine | mică | mici |
Here mici covers both the masculine and feminine plural, so you see only three distinct spellings — but mic still belongs to the four-form class, because it makes a full masculine/feminine distinction in the singular (mic vs mică). Contrast this with a true three-form adjective like mare, which collapses the genders in the singular too.
un copil mic și o fetiță mică
a small child and a small little girl (mic vs mică in the singular)
copii mici și fetițe mici
small children and small little girls (mici for both plurals)
Common Mistakes
The o→oa shift is the number-one stumbling block. Don't keep the masculine o in the feminine:
❌ o zi frumosă
Incorrect — the -ă ending forces o → oa: frumoasă.
✅ o zi frumoasă
a beautiful day
Don't leave the consonant unchanged before the masculine plural -i:
❌ băieți înalti
Incorrect — t softens before -i: înalți, not înalti.
✅ băieți înalți
tall boys
The all-purpose English-speaker error: freezing the masculine singular on a feminine noun:
❌ o casă bun
Incorrect — a feminine singular noun needs bună.
✅ o casă bună
a good house
Don't use the feminine plural ending on a masculine plural noun:
❌ prieteni bune
Incorrect — masculine plural is buni; bune is feminine plural.
✅ prieteni buni
good friends (male/mixed)
Key Takeaways
- Four-form adjectives distinguish all four cells: consonant / -ă / -i / -e = m.sg / f.sg / m.pl / f.pl.
- A stressed o becomes oa before the feminine -ă/-e endings: frumos → frumoasă.
- The masculine plural -i softens or changes the final consonant exactly as in noun plurals: înalt → înalți, prost → proști.
- These are the same alternations as in noun plurals — the two systems are one.
- Some four-form adjectives (like mic) show only three distinct spellings, but still belong to this class because they split the genders in the singular.
Now practice Romanian
Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.
Start learning Romanian→Related Topics
- Romanian Adjectives: An OverviewA1 — How Romanian adjectives agree with their noun in gender and number and normally follow it, with a preview of the four-form, three-form, two-form, and invariable classes.
- Three-Form, Two-Form, and Invariable AdjectivesA2 — Romanian adjectives that distinguish fewer than four forms — mare/mari, verde/verzi — and the invariable loan-colors roz, bej, maro, gri that never change at all.
- Masculine Plurals (-i)A2 — Romanian masculine nouns form their plural with a single ending — -i — but that -i triggers palatalization of the final consonant (brad→brazi, perete→pereți, urs→urși), and the audible change is in the consonant, not the often-whispered final -i.
- Feminine Plurals (-e, -i)A2 — Feminine plurals are Romanian's trickiest: the ending splits between -e and -i, and a root-vowel shift (a→e in masă→mese, oa→o in poartă→porți, a→ă in carte→cărți) usually fires at the same time. This same plural stem is what the feminine genitive-dative singular is built on.
- Mistake: Adjective and Article AgreementA2 — English speakers leave adjectives frozen in the masculine-singular dictionary form (*o casă mic) and double-article fronted adjectives (*frumoasa fata). Two habits fix almost everything: always inflect the adjective to match its noun, and put the definite article on the FIRST element only.