What's the difference between 快 and 快要? I started to learn Chinese like one month ago and I'm facing a first misunderstanding. 

In my book it says that 快 can be used to say "soon" but 快要 is also used to express the same thing. Is there a difference or I can use both in every situation?
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.