Four-Form Adjectives (bun, bună, buni, bune)

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

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:

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.

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:

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.

Careful: mic looks four-form, but it isn't

Because a four-form adjective splits gender in the singular, learners often assume that any adjective which does so — like mic ("small"), mic vs mică — belongs here. But the class is defined by the number of distinct forms, and mic comes up one short:

In the singular mic behaves just like bun (mic / mică), but its two plurals fall together into a single mici. That leaves only three distinct forms — mic / mică / mici — so mic is a three-form adjective, not a four-form one. A true four-form adjective keeps all four cells distinct (bun / bună / buni / bune, frumos / frumoasă / frumoși / frumoase); the moment two of them coincide, the adjective drops into a smaller class.

Common Mistakes

The o→oa shift is the number-one stumbling block. Don't keep the masculine o in the feminine:

Don't leave the consonant unchanged before the masculine plural -i:

The all-purpose English-speaker error: freezing the masculine singular on a feminine noun:

Don't use the feminine plural ending on a masculine plural noun:

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.
• An adjective that splits gender in the singular but merges its plurals (mic → mici) has only three distinct forms — it is a three-form adjective, not four-form; the four-form class keeps all four cells distinct.