Simple
I think you can't have a subordinate independent clause. A subordinate clause is a clause which is dependant on another clause it can't stand alone as a sentence. An independent clause can stand alone as a sentence.
The sentence type that includes both a subordinate clause and an independent clause would be a complex sentence. You would need to insert a conjunction to separate the two clauses for the sentence to be grammatically correct.
Simple
Complex (APEX)
Complex (APEX)
,and ,or ,but
The sentence "That was the reason why he came late" is a complex sentence, as it contains an independent clause ("That was the reason") and a dependent clause ("why he came late").
Not a simple sentence, but a compound, complex or compound-complex may have its principal or independent clause in inverted order within the sentence.
An independent clause contains a subject and a verb. It need contain nothing else, though it may. If the verb is impersonal, it need not even contain a subject. 'It is snowing' is an independent clause consisting of a three-word impersonal verb formation and nothing else.
The clause "these sentences are not very difficult" in the given sentence is an independent clause because it can stand alone as a complete sentence and expresses a complete thought.
"Before" is a subordinating conjunction that introduces a dependent clause. It is used to show the relationship between the dependent clause and the independent clause in a sentence.
"Whenever" is a subordinating conjunction.