Language Arts for Kids/grammer
"Rainsford knows that this island is feared by every sailor who passes by.
what is the verb, adverb, subject, and prepositional phrase
This is a very complicated sentence.
Rainsford knows is the subject verb. Knows 'what' is usually how you would find a direct object and in this case the rest of the sentence fits that definition.
The adverb also is identified by looking at knows. You have to ask knows how, in what way or to what degree...this sentence does not answer that question. An example is John dresses very successfully for his job. Her very successfully describes how John dresses. I don't see that in your sentence. Rainsford might know how the island was feard by studying old texts...but we don't have anything like that.
The only preposition in the sentence is 'by', but again it is used in an unusual way. Prepostitions usually identify direction, location or time...to the north, by the lake, after dinner. 'by every sailor' doesn't fit the rule.
What you have is a string of clauses.