An adverbial noun is a word which serves the function of either a noun or an adverb depending on the sentence in which it is used. Nouns dealing in measurements of some kind, such as time frames or distances, are usually adverbial nouns. Like adverbs, these nouns normally modify verbs but can also modify adjectives.
As modifiers, adverbial nouns provide additional details about a verb or adjective. They answer a question of measurement, such as the distance, direction, or time of an action or the weight or value of a thing. Although these nouns often occur at or near the end of a sentence, they should not be confused with the object of the sentence. The object refers to the thing the action is acting upon or happening to and thus answers the question "what?"
For example, in the sentence "I drove north," the word "north" is an adverbial noun. It is giving additional detail about the direction in which speaker drove and answering the question of "where." Likewise, "I drove for an hour" indicates a length of time, answering "how long?," making "an hour" also an adverbial noun. The sentence "I drove my car," however, answers a question of "what." The words "my car" are, therefore, the object of the sentence.
Nouns may remain unchanged when used as adverbs, or they may have their own adverbial forms. Just as with adjectives, adverbial forms of nouns commonly end in the suffix "-ly". For example, in the sentence "the festival is held yearly," the word "yearly" is the adverbial form of the word "year." The word "year" can be used as an adverbial noun without the "-ly" suffix as well: "She has worked a year."
Nouns denoting numbers and seasons are also commonly used for adverbs. For example, "she lifted 20 pounds." The words "20 pounds" tell the audience how much she lifted, not what she lifted, so they are acting as adverbial nouns. Likewise, in the sentence, "Heidi starts school this autumn," the word "autumn" serves as an adverbial noun. This example also contains an object: "school."
Adverbial nouns can also modify an adjective in a sentence. The adjective "worth," for example, is usually modified by one of these nouns. In the sentence, "the necklace is only worth a dollar," the word "dollar" is an adverbial noun. Since worth indicates an answer to the question "how much," it requires an amount as a modifier.