As
Did the title catch you off guard? Yes, today we are talking about one word—"as;" if I only had a penny. . . .
This word is extremely diverse. Consequently, our English-speaking minds will latch onto the principle of analogy and we start to write it everywhere. I often see this word where it should not be. I would like to, therefore, rehearse a brief overview of instances where "as" is appropriate.
As maybe used as an adverb, conjunction, or preposition.
Adverb
For the sake of simplicity, think of adverbs as "extra information." You have been taught to recognize these words by ending in the English suffix "-ly," but they do not have to. They answer questions such as "how?" "when?" "where?" "how much?" "how often?" and so forth. Enough grammar.
This is probably the usage with which you are most familiar. When "as" is an adverb, it compares:
Paul's view of the kingdom was the same as that of Jesus.
The Verb יחל can mean "to wait on; to hope" as can the verb קוה.
Students do not have too much trouble with the usage.
Conjunction
A conjunction joins stuff together. This connection may be a simple joining of two items/clauses which have equal syntactic weight; we use and for this:
The Bible consists of the Old and New Testaments.
John Calvin's early life took place in a setting of humanist progressivism and religious tension.
Most of the time, however, conjunctions subordinate one clause to another. "As" falls within this category. "As" can:
Temporally subordinate, like the word while:
As Jesus was praying, His disciples fell asleep.
The Romans crucified the Messiah as the Jews looked on.
Indicate manner (In the manner of):
Paul writes to the churches as their apostle (i.e., in the manner of their apostle).
The readers of 1 Peter surely took the letter as consolation (i.e., in the manner of consolation).
Stand for the word "because" or "since":
Uthmann burned nonhomogenous manuscripts of the Quran as (since) individual interpretations among the Arab tribes caused political division and turmoil.
As (because) both Fmin6 and Gmaj7 have an equal intervalic relation to the key center, they may both resolve to C.
Stand for the word "although" or "even though":
As great as it is (i.e., even though it is great), I did not like that movie.
Preposition
A preposition describes spacial relationships whether physically or metaphorically. In this respect, you may think of "as" with the phrase "in the role/function of":
The passover lamb functions as a typological figure to the death of Christ
Incorrect Uses of As
Please please do not use "as" like you would the relative pronouns "which" or "that": The ball, as the boy kicked, was red. The correct usage would be: "The ball, that or which the boy kicked, was red.
Please do not use as + a noun to state a comparison (like): The boy kicked the rock as a ball. For a comparison, you must use "as" + a clause: The boy kicked the rock as he would have the ball.
Likewise, please do not use as + a clause to indicate role/function. What if you were trying to say that Paul was called to the role of an apostle. "Paul was called as an apostle is called." This is incorrect; this sentence tells me that the way Paul was called was similar to that of an apostle. This is true, but not if you are trying to say that Paul was called to the role of an apostle. This sentence would look like: "Paul was called as an apostle."
Conclusion
A teacher of mine used to say "that which is most common is most irregular." The word "as" is not irregular, but this aphorism does support the breadth of semantic usages a common word like "as" can cover. This may seem nit-picky but, unless you are sure of the environment in which you write "as," it can become very difficult for your reader to understand your logic! I sincerely hope paranoia now creeps upon with each composition of this word :).
Resources for further reference (and which were referenced for this post):
http://www.macmillandictionary.com/dictionary/british/as_1
https://en.oxforddictionaries.com/definition/as
https://gmatclub.com/forum/as-vs-like-correct-and-incorrect-usages-133950.html