In the sentence, "He is as sly as a fox", what part of speech is "as"? Is "as sly as a fox" an adjective? Is "sly" the object of a preposition? I am so confused. Thank you!

The word "as" is a conjunction in this sentence; in fact, the two together are being used as correlative conjunctions. See the first example under conj. below:

http://www.answers.com/as

The word "sly" is an adjective. The word "fox" is a noun. Taken all together, it's a simile.

http://www.google.com/search?q=idiom+%22as+sly+as+a+fox%22&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7SUNA

In the sentence "He is as sly as a fox," the word "as" is functioning as a conjunction. It is used to compare the level of slyness between "he" and "a fox." The phrase "as sly as a fox" is known as a simile, which is a figure of speech that compares two similar things using "like" or "as."

To understand the parts of speech in this sentence, let's break it down:
- "He" is a pronoun functioning as the subject of the sentence.
- "Is" is a linking verb that connects the subject "he" to the complement "as sly as a fox."
- "As" is a conjunction that introduces the comparison between "he" and "a fox."
- "Sly" is an adjective describing the degree or quality of "he," and it is not the object of any preposition.

So to summarize, "as" is not an adjective, but rather a conjunction that introduces a comparison, while "sly" is an adjective describing the subject "he" and is not the object of a preposition in this sentence.