How much full is the class...

[MeyaN]

"How full is the class?"
"How much full is the class?"
"How much if the class is occupied?"

Please tell me if all the above sentences are correct.

 

[JMurray]

Not a teacher.

"How full is the class?"
"How much full is the class?"
"How much of the class is occupied?"

1) The first example is correct.
2) The second one is incorrect.
3) The third is not natural and rather than the meaning you intend, it suggests the question, "How many in the class are busy?".

Note the correction to the third example; 'of' not 'if'.

 

[MeyaN]

Please tell me why the second sentence is incorrect.