The Collaborative International Dictionary of English v.0.48:

If \If\, conj. [OE. if, gif, AS. gif; akin to OFries. ief, gef,
   ef, OS. ef, of, D. of, or, whether, if, G. ob whether, if,
   OHG. oba, ibu, Icel. ef, Goth. iba, ibai, an interrogative
   particle; properly a case form of a noun meaning, doubt (cf.
   OHG. iba doubt, condition, Icel. if, ef, ifi, efi), and
   therefore orig. meaning, on condition that.]
   1. In case that; granting, allowing, or supposing that; --
      introducing a condition or supposition.
      [1913 Webster]

            Tisiphone, that oft hast heard my prayer,
            Assist, if [OE]dipus deserve thy care. --Pope.
      [1913 Webster]

            If thou be the Son of God, command that these stones
            be made bread.                        --Matt. iv. 3.
      [1913 Webster]

   2. Whether; -- in dependent questions.
      [1913 Webster]

            Uncertain if by augury or chance.     --Dryden.
      [1913 Webster]

            She doubts if two and two make four.  --Prior.
      [1913 Webster]

   As if, But if. See under As, But.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

But \But\ (b[u^]t), prep., adv. & conj. [OE. bute, buten, AS.
   b[=u]tan, without, on the outside, except, besides; pref. be-
   + [=u]tan outward, without, fr. [=u]t out. Primarily,
   b[=u]tan, as well as [=u]t, is an adverb. [root]198. See
   By, Out; cf. About.]
   1. Except with; unless with; without. [Obs.]
      [1913 Webster]

            So insolent that he could not go but either spurning
            equals or trampling on his inferiors. --Fuller.
      [1913 Webster]

            Touch not the cat but a glove.        --Motto of the
                                                  Mackintoshes.
      [1913 Webster]

   2. Except; besides; save.
      [1913 Webster]

            Who can it be, ye gods! but perjured Lycon? --E.
                                                  Smith.
      [1913 Webster]

   Note: In this sense, but is often used with other particles;
         as, but for, without, had it not been for. "Uncreated
         but for love divine." --Young.
         [1913 Webster]

   3. Excepting or excluding the fact that; save that; were it
      not that; unless; -- elliptical, for but that.
      [1913 Webster]

            And but my noble Moor is true of mind . . . it were
            enough to put him to ill thinking.    --Shak.
      [1913 Webster]

   4. Otherwise than that; that not; -- commonly, after a
      negative, with that.
      [1913 Webster]

            It cannot be but nature hath some director, of
            infinite power, to guide her in all her ways.
                                                  --Hooker.
      [1913 Webster]

            There is no question but the king of Spain will
            reform most of the abuses.            --Addison.
      [1913 Webster]

   5. Only; solely; merely.
      [1913 Webster]

            Observe but how their own principles combat one
            another.                              --Milton.
      [1913 Webster]

            If they kill us, we shall but die.    --2 Kings vii.
                                                  4.
      [1913 Webster]

            A formidable man but to his friends.  --Dryden.
      [1913 Webster]

   6. On the contrary; on the other hand; only; yet; still;
      however; nevertheless; more; further; -- as connective of
      sentences or clauses of a sentence, in a sense more or
      less exceptive or adversative; as, the House of
      Representatives passed the bill, but the Senate dissented;
      our wants are many, but quite of another kind.
      [1913 Webster]

            Now abideth faith hope, charity, these three; but
            the greatest of these is charity.     --1 Cor. xiii.
                                                  13.
      [1913 Webster]

            When pride cometh, then cometh shame; but with the
            lowly is wisdom.                      --Prov. xi. 2.
      [1913 Webster]

   All but. See under All.

   But and if, but if; an attempt on the part of King James's
      translators of the Bible to express the conjunctive and
      adversative force of the Greek ?.
      [1913 Webster]

            But and if that servant say in his heart, My lord
            delayeth his coming; . . . the lord of that servant
            will come in a day when he looketh not for him.
                                                  --Luke xii.
                                                  45, 46.
      [1913 Webster]

   But if, unless. [Obs.] --Chaucer.
      [1913 Webster]

            But this I read, that but if remedy
            Thou her afford, full shortly I her dead shall see.
                                                  --Spenser.
      [1913 Webster]

   Syn: But, However, Still.

   Usage: These conjunctions mark opposition in passing from one
          thought or topic to another. But marks the opposition
          with a medium degree of strength; as, this is not
          winter, but it is almost as cold; he requested my
          assistance, but I shall not aid him at present.
          However is weaker, and throws the opposition (as it
          were) into the background; as, this is not winter; it
          is, however, almost as cold; he required my
          assistance; at present, however, I shall not afford
          him aid. The plan, however, is still under
          consideration, and may yet be adopted. Still is
          stronger than but, and marks the opposition more
          emphatically; as, your arguments are weighty; still
          they do not convince me. See Except, However.
          [1913 Webster]

   Note: "The chief error with but is to use it where and is
         enough; an error springing from the tendency to use
         strong words without sufficient occasion." --Bain.
         [1913 Webster]