The part of the grammar page this lesson drills — read it here, then use it.
Telling the hour
The count noun сат follows the same 1 / 2–4 / 5+ pattern as numbers-a1 - see the exact forms and verbs in the table below.
That је/су switch is the one detail worth memorizing on purpose: 2–4 pairs with plural су, while 1 and 5+ both pair with singular је - an asymmetry that has nothing to do with real-world plurality.
Minutes, half and quarter hours
Minutes past the hour are added with и (and): пет и десет (5:10), пет и петнаест or пет и четврт (5:15, четврт = quarter).
Пола (half) needs care: пола шест means 5:30, not 6:30 - it points at the UPCOMING hour, literally half [of the way to] six. Четврт до (quarter to) works the same way: четврт до шест means 5:45.
Special cases
To say at a time, use у + accusative: Долазим у пет сати. (I'm arriving at five o'clock.) The question form is a fixed phrase: У колико сати долазиш? (What time are you arriving?).
Подне (noon) and поноћ (midnight) are fixed words on their own, not a number + сат: Сада је подне. (It's noon now.) Formal schedules (trains, TV listings) commonly switch to the 24-hour clock, e.g. у 17 и 30.