noun +of +noun, noun+'s+ noun, or noun+noun? is there any rule?
using a noun(1) describe another noun(2), I know three ways: inserting an 'of' between the two nouns, plus 's after noun(1), and puting noun(1) before noun(2). Such as:

the top of the mountain;
the mountain's top;
the mountain top;

or:

the level of English
the English's level
the English level

Are all of the three forms right?

Question

noun +of +noun, noun+'s+ noun, or noun+noun? is there any rule?
using a noun(1) describe another noun(2), I know three ways: inserting an 'of' between the two nouns, plus 's after noun(1), and puting noun(1) before noun(2). Such as:

the top of the mountain; 
the mountain's top;
the mountain top;

or:

the level of English
the English's level
the English level

Are all of the three forms right?

Answer

No, they're not. You just have to learn which ones are OK and which ones aren't.

English's level = level of THE English, and therefore it is wrong.

Watch your emphasis!
The mountain's TOP.
The MOUNtain top.