1. Where are all the boys? 
 
What is the sentence about?  The
boys (subject)
Which boys? All the boys (more info about the boys)
Verb? Are   
More info about the verb? Time place manner?  Where (predicate) 
 
 All the boys/ are / where
 Subject    / verb / predicate
 
2.How dirty his clothes are? Do you mean: How dirty
are his clothes?
 
What is the sentence about?  Clothes
Whose clothes? His clothes
Verb? Are
More about the verb? Dirty
More about dirty? How dirty
 
 His clothes / are / how dirty
 Subject    /
verb / predicate