A phrase or a clause used as an adverbial modifier is called an adverbial phrase or clause.
Adjective and adverbial clauses are always subordinate, because they are used as parts of speech ([§ 46]).
MODIFIERS OF THE SUBJECT
466. Any substantive in the sentence may take an adjective modifier, but modifiers of the subject are particularly important.
The simple subject may be modified by (1) an adjective, an adjective phrase, or an adjective clause; (2) a participle; (3) an infinitive; (4) a possessive; (5) an appositive.
I. ADJECTIVES, ADJECTIVE PHRASES, ADJECTIVE CLAUSES
467. The simple subject may be modified by an adjective, an adjective phrase, or an adjective clause.
- {Ivory trinkets | Trinkets of ivory | Trinkets which were carved from ivory} lay scattered about.
- {Treeless spots | Spots without trees | Spots where no trees grew} were plainly visible.
In each of these groups of sentences, the subject of the first sentence is modified by an adjective, that of the second by an adjective phrase, that of the third by an adjective clause.
Most adjective phrases are prepositional ([§ 42]), as in the examples.