Use of 'of' in particular sentences

[filiphasit]

Hey, guys!

I'm a brazilian english language student, and new to this forum. I have a question, but actually this question is so problematic to me, that I can even explain it in a few words. I'll try.

I see the structure of such sentences in two different ways. I would like to understand when to use each one of them, and whether they are interchangeable, and if one is more formal than the other. Let's go to the examples:

"Please, give me a piece of pie"./"Please, give me a pie piece".

"A rain drop fell over me"./"A drop of rain fell over me".

And I could list other examples:

"Empire state/state of empire".
"Chocolate cake/cake of chocolate".
"Glass of milk/milk glass".
"Sheet of paper/paper sheet".

And so on.
That's it. I haven't been able to figure this out for a long time. I'd really appreciate your help, guys.
Thanks in advance!

[Alan]

I'm afraid that there is no generalized rule to account for the use of nouns as modifiers/adjectives.

If a particular noun can be used in this way, a good learners' dictionary should carry the necessary information with examples.