Subjunctive

     [This is a fast-access FAQ excerpt.]
Present Subjunctive
   The present subjunctive is the same in form as the infinitive
without "to".  This is also the same form as the present indicative,
except in the third person singular and in forms of the verb "to
be".
The present subjunctive is used:
(1) in third-person commands:  "Help, somebody save me!"  Most third-
    person commands (although not those addressed to "somebody") are
    now expressed with "let" instead.  The following (current but
    set) formulas would probably use "let" if they were being
    coined today:  "So be it"; "Manners be hanged!"; "... be
    damned"; "Be it known that..."; "Far be it from me to...";
    "Suffice it to say that..."
(2) in third person wishes.  Most third-person wishes are now
    prefixed with "may" instead, as would the following formulas be:
    "God save the Queen!"; "God bless you"; "God help you"; "Lord
    love a duck"; "Hallowed be thy name.  Thy kingdom come.  Thy
    will be done."; "Heaven forbid!"; "The Devil take him!"; "Long
    live the king!"; "Perish the thought!"
(3) in formulas where it means "No matter how..." or "Even if...":
    "Come what may, ..."; "Be that as it may, ..."; "Though all
    care be exercised..."; "Be he ever so..."
(4) after "that" clauses to introduce a situation that the actor
    wants to bring about.  Used to introduce a formal motion ("I move
    that Mr Smith be appointed chairman"); after verbs like
    "demand", "insist", "propose", "prefer", "recommend", "resolve",
    "suggest"; and after phrases like "it is advisable/desirable/
    essential/fitting/imperative/important/necessary/urgent/vital
    that".  "Should" can also be used in such clauses.  This use of
    the subjunctive had become archaic in Britain in the first half
    of the 20th century, but has been revived under U.S. influence.
    Note the difference between "It is important that America has
    an adequate supply of hydrogen bombs" (America has an adequate
    supply of H-bombs, and this is important) and "It is important
    that America have an adequate supply of hydrogen bombs" (America
    probably *lacks* an adequate supply, and must acquire one).
(5) after "lest".  "Should" can also be used after "lest".  After
    the synonymous "in case", the plain indicative is usual.
(6) "Come...", meaning "When ... comes"
Past Subjunctive
   The past subjunctive is the same in form as the past indicative,
except in the past subjunctive singular of "to be", which is "were"
instead of "was".
The past subjunctive is used:
(1) for counterfactual conditionals:  "If I were..." or
    (literary) "Were I...".  In informal English, substitution of
    the past indicative form ("If I was...") is common.  But note
    that speakers who make this substitution are *still*
    distinguishing possible conditions from counterfactual ones,
    by a change of tense:
                Present         Past
    Possible condition:         "If I am"       "If I was"
    Counterfactual condition:   "If I were/was" "If I had been"
    "As if" and "as though" were originally always used to introduce
    counterfactuals, but are now often used in "looks as if",
    "sounds as though", etc., to introduce things that the speaker
    actually believes ("It looks as if" = "It appears that").  In
    such cases the present indicative is often used.  ("As if" and
    "as though" are exceptions to the above table in that they take
    the past subjunctive, not the pluperfect subjunctive, for
    counterfactuals in the past.  The past tense of "If he were a
    fool, he would mention it" is "If he had been a fool, he would
    have mentioned it"; but the past tense of "He talks as if he
    were a fool" is "He talked as if he were a fool."  "He talked as
    if he had been a fool" would mean that he seemed, not foolish,
    but regretful of earlier foolishness.)
    Fowler says that there is no "sequence of moods" requirement in
    English:  it's "if I were to say that I was wrong", not "if I
    were to say that I were wrong".
(2) for counterfactual wishes:  "I wish I were...";  "If only I
    were..."; (archaic) "Would that I were...".  Again, substitution
    of the past indicative is common informally.  Achievable wishes
    are usually expressed with various verbs plus the infinitive:
    "I wish to...", "I'd like you to..."
(3) in archaic English, sometimes to introduce the apodosis
    ("then" part) of a conditional:  "then I were" = "then I would
    be".
(4) in "as it were" (a formula indicating that the previous
    expression was coined for the occasion or was not quite
    precise -- literally, "as if it were so").
    [See Neil Coffey's comments on the subjunctive.]