What time is it?

시 is a counter, like 개 and 명 on day 9, and counting is the job of the native numbers. So 세 시 is literally 'three hours' on the dial, counted the way you count three cups or three people. The same shortened forms apply: 한, 두, 세, 네, and 열한, 열두 at the top of the clock. Say 삼 시 and you will be understood, but it marks you out at once as a learner who has picked the wrong set.

The rule of thumb is native numbers for counting and Sino numbers for measuring. The hours on a clock face are counted like objects, but minutes, like the won on day 10 and the phone digits on day 8, are a measured quantity, so they go with the Sino set. The mix is not an exception to work around; every time with minutes in it uses both systems at once. Phone screens and timetables show the digits, 4:20, and after a week of practice you will read the two halves in two systems without thinking.

에 marks the point in time when something happens, just as it marked a point in space on days 15 and 16. English switches between 'at six', 'on Monday' and 'in June'; Korean uses 에 for all three, as you will see tomorrow. The order, 오후 세 시 삼십 분, runs from the biggest unit to the smallest, and that is the Korean habit everywhere, in dates and addresses too: set the widest frame first and narrow down.