Telling the time in English

Could you tell me which one is wrong or unnatural among the following sentences?

1. It is about ten o’clock.
2. It is five past ten.
3. It is ten past ten.
4. It is a quarter past ten.
5. It is two quarters past ten. No
6. It is half past ten.
7. It is two quarters to eleven.

Number 8 i definitely wrong. Number 7 seems to be wrong. Instead you can say "It is 10.30". Similarly number 5 can be said "It is 10.30". Number 4 sounds wrong..

4 is not wrong.

You can also say "it's a quarter to 11" to mean it's 10:45

How about "It is fifteen past ten" or "It is fifteen to ten"?

No.

Some people might be confused by the fact that, in informal spoken BrE, 'half ten' means 'half past ten, 10.30'.

Do you say "It's one five" instead of "It's five past one"? Also, are all of "almost/about/around" commonly used to tell time approximately, for example "It's [ almost/about/around ] ten twenty"?

In the US, we say "It's one oh five" if the clock shows 1:05.

12:20 It's twelve twenty
9:18 It's nine eighteen
3:10 It's three ten

If you have a clock or watch with hands, it's common to say something like "it's about/around ten twenty" if the minute hand is near but not exactly on the 4. Obviously, you'd only use "almost" if the minute hand had not quite reached the 4.

Thank you for your help, BarbaraPA. By the way, is "It's one oh five" just preferred, or do American people say only that way?

It's one oh five
It's five after one
It's five past one

All are fine.

12:55
It's twelve fifty-five
It's five minutes to one
It's five to one
It's five of one -- this one is not universal - it seems to be highly regional, but it was common where I grew up.