"like" vs "as"

     [This is a fast-access FAQ excerpt.]
For making comparisons (i.e., asserting that one thing is similar
to another), the prescribed choices are:
   1.  A is like B.
   2.  A behaves like B.
   3.  A behaves as B does.
   4.  A behaves as in an earlier situation.
In 1 and 2, "like" governs a noun (or a pronoun or a noun phrase).
In 3, "as" introduces a clause with a noun and a verb.  In 4, "as"
introduces a prepositional phrase.  Look at what the word
introduces, and you will know which to use.
   In informal English, "like" is often used in place of "as" in
sentences of type 3 and 4.  "Like" has been been used in the sense
of "as if" since the 14th century, and in the sense of "as" since
the 15th century, but such use was fairly rare until the 19th
century, and "a writer who uses the construction in formal style
risks being accused of illiteracy or worse" (AHD3).  "Like" in 1
and 2 is a preposition; "as"/"like" in 3 or 4 and "as if" are
conjunctions.  Fowler put "Like as conjunction" first in his list
of "ILLITERACIES" (he defined "illiteracy" as "offence against the
literary idiom").
   In some sentences of type of 3, "as" may sound too formal:
"Pronounce it as you spell it."  To avoid both this formality and
the stigma of "like" here, you may use "the way":  "Pronounce it the
way you spell it."  But this solution is available only if you are
specifying a single way; it doesn't work, for example, in "Play it
as it's never been played before."  ("Play it in a way..." might
work here, but lacks the connotations of enthusiasm and excellence
that "play it as" has.)
   The most famous use of "like" as a conjunction was in the 1950s
slogan for Winston Cigarettes:  "Winston tastes good, like a
cigarette should."  The New Yorker wrote that "it would pain [Sir
Winston Churchill] dreadfully", but in fact conjunctive "like" was
used by Churchill himself in informal speech:  "We are overrun by
them, like the Australians are by rabbits."  "Like" in the sense of
"as if" was, until recently, more often heard in the Southern U.S.
than elsewhere, and was perceived by Britons as an Americanism.
When used in this sense, it is never now followed by the inflected
past subjunctive:  people say "like it is" or "like it was", not
"like it were".
   Sometimes, "as" introduces a noun phrase with no following verb.
When it does, it does not signify a qualitative comparison, but
rather may:
a) indicate a role being played.  "They fell on the supplies as men
starving" means that they were actually starving men; in "They fell
on the supplies like men starving", one is *comparing* them to
starving men.  "You're acting as a fool" might be appropriate if you
obtained the job of court jester; "You're acting like a fool"
expresses the more usual meaning.
b) introduce examples.  ("Some animals, as the fox and the squirrel,
have bushy tails.")  "Such as" and "like" are more common in this use.
For the use of "like" here, see the next entry.
c) be short for "as ... as":  "He's deaf as a post" means "He's as
deaf as a post" (a quantitative comparison).