Locate the subordinate clause in the following sentence, and identify it as a noun, adjective, or adverb.

i had root beer in my cereal this morning because i forgot to buy milk.

The subordinate clause in this sentence is because I forgot to buy milk. The whole sentence is used as an adverb. Adverbs are used to answer questions such as where, when, or in this case, why. 
Why did I have root beer in my cereal? Because I forgot to buy milk.