This is based on my understanding. Others can correct or change it if necessary. Adverbial clauses add information such as how, when, how much, or why an event took place. It has the same use as an adverb and adds more information about a VERB. Adverbials can usually be moved in the sentence, it the sentence will still be grammatical. Example: BEFORE SHE WENT SHOPPING, she did her homework. She did her homework, BEFORE SHE WENT SHOPPING. “Before she went shopping” adds more information about when she did her homework. It functions as an adverbial clause. Attributive clauses add more information about a NOUN. There are restrictive and non-restrictive attributive clauses. A restrictive attributive clause adds more information about the noun to determine which noun specifically is meant. Example: Please give me the book that is blue. “That is blue” adds more information to the noun book so we know which book to pass. This would be said if there is more than one book. "That" is used with restrictive attributive clauses and never the relative pronoun "which". When referring to a person, some speakers of English use "who" in restrictive attributive clauses and never "that", however, some speakers of English also use "that" when referring to a person. It depends on the dialect. A non-restrictive attributive clause adds also more information to a noun, but we already know which specific noun is implied, either by context or logic. Example: I visited the Eiffel Tower, which is 324m tall. “Which is 324m tall” does not help me determine which Eiffel Tower because there is only one. (Unless you count other replicas of the Eiffel Tower, in that case you would use a restrictive attributive clause such as: I visited the Eiffel Tower that is in Las Vegas) Non-restrictive clauses use relative pronouns such as "who" and "which". Some speakers of English also use "that" but it depends on the dialect.