There are two kinds of numbers, cardinal and ordinal number. Cardinal numbers are used pretty much for everything, ordinal numbers are used on dates and to organize in order of first, second and so on.
When we are talking about time there are some different possibilities we can say the time, for example if it´s 10:15 it´s possible to say it´s ten fifteen, it´s fifteen past ten or it´s a quarter past ten. We can use quarter because an hour has 60 minutes so 15 minutes is the same as a quarter of 60 minutes. Between 1 minute and 30 minutes we say past the hour it is in. so we say 10:20 it´s twenty past ten, 10:30 it´s thirty past ten. From 31 minutes to 59 minutes we use to the hour it´s going to be. For example, 10:35 it´s twenty-five to eleven, 10:45 it´s a quarter to eleven or it´s fifteen to eleven. When it´s rounded time it´s correct to say o´clock, for example 10:00 it´s ten o´clock. 11:00 it´s eleven o´clock.
So as we can see there are most of the times 3 options to say the time. All of them are right but what are the most common ones? The most common are to use past and to the hour. Let’s check it.
10:00 – ten o´clock
10:15 – it´s ten fifteen, it´s fifteen past ten. The most common – it´s a quarter past ten.
10:20 – it´s ten twenty. The most common – it´s twenty past ten
10:30 – it´s ten thirty. The most common – it´s half past ten
10:45 – it´s ten forty-five, it´s fifteen to eleven. The most common – it´s a quarter to eleven.
I hope that now it´s more clear on how to say the time in English