American English : telling the time

It's ten o'clock okay
It's ten oh five okay or It's five after 10.
It's ten fifteen okay
It's ten forty okay or It's 20 'til 11.

and

It's five past ten It's 5 after 10
It's a quarter past ten okay or It's 10 fifteen.
It's half past ten It's 10:30
It's twenty to eleven use 'til instead of to
It's a quarter to eleven " "
It's two minutes to eleven " "

Also we say after, not past

If the person knows about what hour it is you can say It's 20 'til. instead of saying It's 20 "til 11.
'til = until

All the phrases you listed will be understood, but my corrections are the way we speak in the Midwest US, which is standard US English.

I am also from the midwest but I will use 'past' just as often as I use 'after'. maybe it comes with reading too many textbooks...

There's a British English term that Americans don't use

"It's half six."

Or something like that. It always confuses me.

"It's 3:00?" 😁

In Canada we usually say after instead of past as well.
We also use to instead of til

Its funny how it varies from country to country.

Thanks to you all. I'd forgotten about half six - and I say it all the time.

half six is 6:30, right?

Like I said I always get confused

half six is six thirty, yes that is correct. When you think about it , it is a strange way to say it.

I try to encourage my students to learn the various ways to tell the time. One week I taught them the British English way and got them to make some clocks. The next week I taught them the American way. So I hope when they hear both ways, they should understand nonetheless.

is it appropriate if I say:" I'll have to go at half" ?
say it is now 12pm and I'll have to go at 12:30pm

No - I'd say I'll have to go at half past, but only if twelve o'clock had just been mentioned - eg :

A : John should arrive at about twelve.
B : Great. I'll have to go at half past, but at least I'll get to see him.

So, you are saying that if I want to tell the time 12.30pm and the listener knows the hour, I can say that it's 30 till (instead of 30 till 1) but not half (half one or half past twelve) ?

Does that mean that even if the listener knows the hour, I can't say that " I'll have to go at half?" to mean that " I'll have to go at half one" as what "I'll have to go 30 till 1" can be replaced with " I'll have to go at 30 till" ?

'30 till' would technically be OK in your example but it's not natural. Naturally I would say, 'I have to go at half-past.'

'XX till' usually isn't used until about 20 minutes till the hour, but yes, if the hour is know you don't need it. For example, you are in a meeting and the meeting started at 2:00. If you need to wrap up the meeting by 2:50, you could say, 'We need to finish up by 10 till.'

(I'm American.)

Yes - the UK equivalent would be : I have to go at ten to. And for earlier times I have to go at twenty past etc. Similarly ...at half past. Not just half by itself