The structure 快要…了 emphasizes that an action is about to happen. We can use it when talking about something spontaneous or about a natural phenomena. For example, "it's going to rain".
In the structure 快要…了, we should put a predicate expressed by a verb in place of the ellipsis. A subject is placed before the structure.
The Dragon Boat Festival is coming soon!
duānwǔ jié kuàiyào lái le
It's going to rain
kuàiyào xià yǔ le
We can use this structure when we don't know when an action will happen.
Therefore, the structure cannot be preceded by an adverbial modifier of time.
For example, we cannot say:
端午节明天快要来了！ "The Dragon Boat Festival is coming tomorrow!".
We can also use either 要…了 or 快…了 instead of 快要…了.
The structure 要…了
The structures 要…了 and 快要…了 have the same meaning and are used in the same way.
We can use the structures 快要…了 and 要…了 to indicate that an action will happen in a few minutes or in a few years.
I'm going back to my country
wǒ yào huíguó le
They're getting divorced
tāmen yào líhūn le
In some cases, 要...了 can be preceded by an adverbial modifier of time.
My sister is getting married next month
xià ge yuè jiějie jyào jiéhūn le
The structure 就要…了
If a sentence has an adverbial modifier of time, 就 is usually placed at the beginning of the structure.
We can use 就要...了 when an action has been scheduled or when we know what time an action is going to take place.
You're going home today
nǐ jīntiān jiù yào huí jiā le
It's going to rain soon
mǎshàng jiù yào xià dàyǔ le
The structure 就…了
We can also use 就…了 instead of 就要…了.
就…了 is used when talking about actions that are going to happen in the near future.
The plane takes off in half an hour
hái yǒu bàn ge xiǎoshí fēijī jiù fēiqǐ le
The structure 快…了
The structure 快…了 can also mean "soon".
In this meaning, 快…了 can only be used in sentences in which a predicate is expressed by either an adjective or a numeral with a measure word.