"Yang" is a word to indicate a specific thing that you're talking about.
"I have a cat." is "Saya punya kucing." "Saya punya kucing hitam." is "I have a black cat." "Saya punya kucing YANG hitam." is "I have a cat WHICH IS black."
A lot of times, you don't actually need to say "yang." (Frankly, half these practice sentences are very, very stilted.) But, if you're in a situation where you need to clarify something, or be specific about which one you're talking about, then YANG is your "the one THAT IS this specific way."
Does that help at all?
(Source: I learned Indonesian by living there. I'm only doing Duo to keep practicing so I don't forget it all.)
To my feeling it is like the الذي word in Arabic, they also have one word for it. In European languages it is translated in 2 words: 'That is'. In Dutch they call it 'die...is'. In French: 'qui est...'
I don't know if it is correct, but this translation fits perfectly in every sentence with 'yang' I've seen until now.
Saya makan apel yang merah: I eat the apple 'yang' red...I eat the apple that is red.
"I want to order regular drink" is not correct English. You need "the" or "a" to make it grammatically correct.
We are learning conversation. In conversation "My Usual" or "The Usual" is more common in speech. The acceptable answers should reflect common speech patterns.
It maybe correct in literal translation to ask for a "Regular" Drink. No-one speaks like this. So terms like, usual and standard which are parts of the common phrase should be accepted. Phil