Adjective agreement in the nominative

In English a describing word never moves: a red coat, a red dress, red coats. Red stays red. In Serbian the describing word copies the thing it describes. Say crven kaput (a red coat) but crvena haljina (a red dress) — same colour, different ending, because kaput and haljina are different genders.

You already read a noun's gender off its ending, and you already know how to make it plural. That is all the information you need here: look at the noun, then put the matching ending on the adjective. Six combinations, six endings.

The six endings

The masculine singular has no ending at all — it is the bare word, and it is the form you look up in a dictionary: crven, plav, nov, star, zelen. Everything else is that bare word plus one letter.

Neuter plural and feminine singular both end in -a: crvena odela (red suits) and crvena haljina (a red dress). Nothing distinguishes the adjectives — the noun tells you which one you are looking at.

One word you already know is a trap here: auto (car) ends in -o but it is masculine. So it takes masculine endings — Auto je star, Auto je plav — never staro, never plavo. A few borrowed words ending in -o behave this way; the ordinary -o nouns are neuter, as you learnt.

Before the noun, and after je

The ending does not care where the adjective stands. Put it in front of the noun, or put it after je / su, and it is the same form either way.

Two things to watch

beo (white) hides an l. The bare masculine is beo, but every other form brings the l back: bela, belo, beli, bele, bela. Learn it as a small exception, not as a pattern.

mali (small) is the one word here you will always meet with an -i on it — there is no bare mal. Its other endings are perfectly ordinary: mala, malo, mali, male, mala.

One last thing, so it does not surprise you later: Serbian also has a longer masculine form — crveni kaput beside crven kaput, veliki grad beside velik grad — used when the thing is one both speakers already have in mind. You do not need it yet; the endings in the table above will carry you everywhere.