Question

the principal is the only one to grant this permission change
into adjective clause​

Answer 1

A relative clause is a kind of subordinate clause that contains the element whose interpretation is provided by an antecedent on which the subordinate clause is grammatically dependent; that is, there is an anaphoric relation between the relativized element in the relative clause and antecedent on which it depends.[1]

Typically, a relative clause modifies a noun or noun phrase,[1] and uses some grammatical device to indicate that one of the arguments within the relative clause has the same referent as that noun or noun phrase. For example, in the sentence I met a man who wasn't there, the subordinate clause who wasn't there is a relative clause, since it modifies the noun man, and uses the pronoun who to indicate that the same "man" is referred to within the subordinate clause (in this case, as its subject).

In many European languages, relative clauses are introduced by a special class of pronouns called relative pronouns,[2] such as who in the example just given. In other languages, relative clauses may be marked in different ways: they may be introduced by a special class of conjunctions called relativizers; the main verb of the relative clause may appear in a special morphological variant; or a relative clause may be indicated by word order alone.[3] In some languages, more than one of these mechanisms may be possible.