What is an adverb clause?  Some basic rules are that an adverb clause will contain a subject and verb. It is a dependent clause.  Like an adverb, an adverb clause provides the answer to the following adverbial questions:  these are how, when, where and why. Therefore, you will find there is always a subordinated conjunction in the clause which prevents it from being a complete thought, or independent clause. While an adverb is just one word that performs this task, an adverbial clause is two or more words which answer the question how, why, where and when.

examples of an adverb clause

Joan was in the cupboard to hide from her Mom.
Why was Joan in the cupboard?  To hide from her Mom – an adverb clause.
We decided to leave the day the new golf course was created.
When did we decide to leave?  The day the new golf course was created – an adverb clause.
The owner of the boat, with the greatest of ease, moved it off the bank.
How did the owner of the boat move it off the bank? With the greatest of ease – an adverb clause.

To sum up: an adverb clause is a dependent clause and like an adverb, an adverb clause answer the questions: How, When, Where and Why. It is always a subordinated conjunction in the clause which prevents it from being a complete thought, or independent clause. While an adverb is just one word that performs this task, an adverbial or adverb clause is two or more words which answer the questions How, Why, Where and When.