The part of the grammar page this lesson drills — read it here, then use it.
Masculine plural: -и, and the -ов-/-ев- infix
Most masculine nouns simply add -и to the consonant-final singular: студент → студенти, пријатељ (friend) → пријатељи. Nothing else changes.
A monosyllabic masculine noun - one syllable in the nominative singular - inserts -ов- or -ев- before that same -и. After a hard final consonant the infix is -ов-: град (city) → градови, мост (bridge) → мостови. After a soft/palatal final consonant (ч, ж, ш, ђ, ћ, џ, љ, њ, ј, ц) the infix is -ев- instead: кључ (key) → кључеви, нож (knife) → ножеви.
Feminine and neuter plural
Feminine nouns ending in -а swap it for -е: жена (woman) → жене, књига (book) → књиге. Feminine nouns ending in a consonant instead - the exception class from noun-gender-a1, like ноћ - take -и: ноћ (night) → ноћи.
Neuter nouns swap their singular -о or -е for -а: село (village) → села, поље (field) → поља. This covers most neuter nouns cleanly, but a small class of nouns ending in -е inserts an -ен- infix instead of a plain swap - име (name) → имена, време (time) → времена - so it isn't quite as exception-free as it looks.
Sibilarization and the fleeting а
When a masculine noun's stem ends in the velar consonants к, г, х, that consonant often softens to ц, з, с before the plural -и - sibilarization: ђак (pupil) → ђаци, бубрег (kidney) → бубрези, орах (walnut) → ораси. For a monosyllabic noun this usually replaces the -ов-/-ев- infix rather than joining it - one-syllable ђак just sibilarizes and adds -и, not *ђакови - but not always: век (century) → векови keeps its к unchanged and takes the infix instead, so a monosyllabic к/г/х-final noun still has to be checked rather than assumed.
Some masculine nouns carry a fleeting а (непостојано а) in their last syllable - a vowel that appears only in the nominative singular to break up an unpronounceable consonant cluster, and disappears once any ending is added: момак (young man) → момци, пас (dog) → пси. Момци shows both changes at once: the fleeting а drops (момак → момц-) and the exposed к sibilarizes to ц before -и.
Irregular plurals to memorize
A handful of very common nouns have a plural that no rule predicts and simply has to be learned as vocabulary: човек (person, man) → људи (people), око (eye) → очи (eyes), ухо (ear) → уши (ears). Очи and уши are relics of the old Slavic dual number, once reserved for naturally paired things.
Дете (child) → деца (children) and брат (brother) → браћа (brothers) are irregular the same way, with a twist: деца and браћа are themselves feminine-singular-shaped collective nouns with special verb agreement, covered in collective-verbal-nouns-b2. For now, just learn the plural word.