Hi, there. I'm happy to shed some light on this.
First of all, an adverb is a word that modifies a verb, an adjective, or another adverb.
One way to create an adverb is to take an adjective and add "-ly" at the end (e.g. "joyously", "readily", "magnanimously", etc.)
Other adverbs common adverbs include the words "very", "well", "too", etc.
An adverb phrase is some kind of phrase (e.g. a preposition phrase, a participial phrase) that acts as an adverb -- that is to say, it modifies a verb, an adjective, or an adverb. For example:He studied for the GMAT with a sense of fury.
The prepositional phrases "with a sense of fury" is an adverbial phrase. It tells us *how* he studied --- it modifies the verb. She is joyful in an unshakable way.
The prepositional phrase "in an unshakable way" is an adverbial phrase. It modifies the adjective "joyful" --- it qualifies *how* joyful she is.
An adverbial clause is a subordinate (i.e. dependent) clause that acts as an adverb. As a clause, it must have a noun + verb, and act as a mini-sentence within the sentence. For example: He runs everyday because he is addicted to runner's high.
The clause "because he is addicted to runner's high" is adverbial clause. It modifies the verb, answering a "why" questions about it. She is as happy as a clam.
The clause "as a clam" is an adverbial clause (the verb is implied --- "as a clam [is]"). It modifies the adjective "happy" --- it tells us how happy she is.
Notice, in all the examples, the adverbial phrases & clauses do not contain adverbs themselves. An adverbial phrase or clause can contain an adverb, but does not have to.
An adjective is a word that modifies a noun.
An adjectival phrase or clause is a phrase or clause that acts as a adjective --- that is, it modifies a noun. The man with the bucket of fish is yelling loudly.
The prepositional phrases "with the bucket of fish" form an adjectival phrase. It modifies the noun "man" --- it tells us "which" man. I am going to buy a car that can accelerate from 0 to 80 mph in 1.5 seconds.
The clause "that can accelerate from 0 to 80 mph in 1.5 seconds" is an adjectival phrase. It modifies the noun "car" --- it clarifies what kind of car is intended.
Notice, in these examples, the adjectival phrases & clauses do not contain adjectives themselves. An adjectival phrase or clause can contain an adjective, but does not have to.
Does all this make sense?
I'd like to recommend that you check out Magoosh
test prep. We have over 200 lessons videos, include grammar lessons that clarify many of these points. Here's a sample SC lesson: http://gmat.magoosh.com/lessons/615-modifiers-i
Let me know if you have any questions on this.
Magoosh Test Prep