What's the difference between 快 and 快要?
There isn't really a difference in the two if you are using it to mean &quot;soon&quot;, except that 快要 is more formal than 快.
For example:

他快死了。
他快要死了。

Both mean:

He is dying soon.

快 is just a shorten form of 快要 when used in such context and it is quite commonly used in speech.