I think yesterday is a noun, because it is definitley not a verb or adjective. It is not an action and it does not describe anything.
Yesterday is a noun in that sentence.
"Yesterday" is an adverb modifying the noun "afternoon".
The word yesterday is an adverb.
Yesterday is an adverb in that sentence.
Walked is a verb.
It's an adverb.
In the sentence "yesterday the leaves hung on the tree," "yesterday" is functioning as an adverb modifying the verb "hung."
"Yesterday" can be an adverb, a noun, or an adjective.ExamplesAdverb: We arrived yesterday.Noun: Yesterday started well. All our yesterdays.Adjective: Yesterday morning
Yesterday he paddled his canoe out into the lake and camped on an island overnight.
She gave a terrific speech yesterday.
In the sentence "Yesterday he played loudly with his two new puppies in their very big backyard," the parts of speech for each word are as follows: "Yesterday" (adverb), "he" (pronoun), "played" (verb), "loudly" (adverb), "with" (preposition), "his" (pronoun), "two" (adjective), "new" (adjective), "puppies" (noun), "in" (preposition), "their" (pronoun), "very" (adverb), "big" (adjective), and "backyard" (noun).
Unbearable is an adjective. It describes something that cannot be tolerated. "Yesterday's heat was unbearable."