I don't really understand the difference. Someone said zai is location, but it is used also in the sentence "Mum and Dad are drinking tea."
是 is used when a piece of information always holds true for the subject.
I am an American.
Even though the person’s nationality might change, at the time of speaking, he wants to say that his current nationality is American. Note that he also describes himself as “American”, which is a noun. Hence, A 是 B means A is B (noun).
在 is used to describe a continuous action.
Dad and Mum are drinking tea.
It shows that Dad and Mum are doing the action of drinking tea. If you said 是, then “drinking tea” would become a noun that is synonymous to “Mum and Dad”, and that makes no sense. Here, A 在 B means A is B (continuous tense of an action).
A similar concept in Spanish is the difference between “ser” and “estar”. Here, the “ser” is 是, and the “estar” is 在.
(Isn't it funny how Chinese tend to say Dad and Mum, whereas I would always say Mum and Dad?)
Mum and Dad are on the patio drinking tea.
If you take away the location 在露台上 that leaves:
Mum and Dad are drinking tea.
But Chinese is not so simple and straightforward， Chinese is very subtle in its nuances. You will often find 是 and 在 mixed up:
What are they doing?
They're drinking tea.
On the right is my Dad. On the left is my Mum.
This all happened on Monday night.
If we think of 'drinking tea' or '在星期一的晚上' as locations in spacetime, then all 在 ever does is locate things, often with the help of 里 or 上。
Someone said zai is location
Yes, it can used to indicate location. For example, 在哪裡? 在哪裡? 不要隱藏你自己。. If you want to reply this kind of question 在哪裡?(In where?) Then you can say 我在[some place]. Here [some place] is a place you are there. For example 我在廁所, 我在這. So you know 在 is used to indicate location.
"Mum and Dad are drinking tea."
在 can also be used to indicate the action which being done continuously. Use 在[something]. Here [something] is the things which being done. So 媽和爸在喝茶(媽和爸在[something], Where [something] is 喝茶). So 喝茶 was the thing which being done.
What is the difference between 是 and 在?
是 is used to indicate somethings is true. You can use [thing]是[other thing], where 是 is used to indicate that [thing] is [other thing]. For example 我是人.(I am human.)([thing]是[other thing], where [thing] is 我, [other thing] is 人). 是 is used to indicate that 我 is 人. 這裡是哪裡([thing]是[other thing], where [thing] is 這裡, [other thing] is 哪裡) 是 is used to indicate that 這裡 is 哪裡. Since 哪裡 is used to ask where and 這裡 have the effect of 是. So 哪裡 is used to ask the location of 這裡(here).
是 is used to indicate a passive state. Drinking tea isn't a passive state, so 是 isn't appropropriate here.
在 is used to indicate a time or location.
When 在 is placed before the verb phrase "to drink tea", 在 takes on the role of the main verb while 喝茶 becomes a time phrase and the sentence's predicate which yields the meaning "Mum and dad at some relevant time drink tea." with a better translation of "Mum and dad are drinking tea."
Sentence structure usually doesn't translate perfectly between languages. 「在」 should not be translated into English as is/are/am, but usually only one of the following:
- in a state of, or the suffix -ing expressing in the midst of doing an action
In the sentence
Mum and Dad
are is omitted in idiomatic Chinese. A word-by-word translation of the Chinese is
Mum and Dad drinking tea.
To see this in steps, first take the original sentence
Mum and Dad are drinking tea.
and re-express it as
Mum and Dad are in a drink tea state.
Rearrange it slightly to get
Mum and dad are in a state of drink tea.
媽媽 (Mum) 和 (and) 爸爸 (dad) 是 (are) 在 (in a state of) 喝 (drink) 茶 (tea)。
Chinese usually omits 是 (is/are/am) when describing a state of action, so
媽媽 (Mum) 和 (and) 爸爸 (dad)
是 (are)在 (in a state of) 喝 (drink) 茶 (tea)。