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Is there any difference between the following sentences?

  1. Please press the bell if necessary.
  2. Please press the bell as necessary.
  3. Please press the bell when necessary.
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up vote 2 down vote accepted

One can review use of if necessary, as necessary, and when necessary via examples drawn from links at ngrams. Some sets of examples are given below, with a summary after each set.

So check all cylinder fins periodically and clean them if necessary. - Townsend
...be willing to see several therapists if necessary in order to find one who is right for you - Norwood
Then take your complaint to the bully's parents and school authorities, and to the police if necessary. - L.H.D.

The above are typical examples of use of if necessary in explaining what action to take if certain conditions arise. It is supposed that the action might or might not be needed, depending on conditions. While as necessary could be used in these examples without particular change in their meanings, such use happens to be rare.

Repair or replace timer as necessary - Powell
Lubricate or replace heat valve as necessary. - Motor

These two examples illustrate use of as necessary in cases where neither of if necessary or when necessary are quite right; specifically, where multiple actions may or may not be done, each dependent on conditions. Note that among numerous surveyed examples for as necessary, it is far more frequently used in a way not of interest here, in sentences like "skilled employees ... should be regarded as necessary employees" (Railway news).

When necessary, the investigator may engage an appropriate specialist - RSFSR codes
The rope is held in the hands to give the critter slack to play on and is dallied around the saddle-horn when necessary to stop the animal when he goes wrong. - Ward
...consideration and approval by the relevant bodies as and when necessary. - Kenya Gazette

In these and other examples, use of when necessary may be prompted by a sense that conditions will arise to make an action necessary; except that in the Kenya Gazette example, as and when necessary is, more or less, a verbose set phrase that could be adequately replaced by any of the simpler if, as or when phrases.

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I would go so far as to say outright that "if" should be used in cases where the condition is possible, but will only occur once; "when" should be used if the condition is likely, but again will only occur once; and "as" should be used if and only if the condition is likely to reoccur. – Marthaª Feb 7 '12 at 20:43

All three read roughly the same to me but if I were to divide them I would do along a spectrum of how often it may be necessary to press the bell. 3 implies (very weakly) pressing once, otherwise whenever would be there in formal language. 2 Implies several presses to my mind. 1 Seems very neutral. Alas I have no source or proof for these gut reactions so don't base any importance on my musings!

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I'll buy that. I think as well as your assessment of how many times the bell may need to be pressed, there's also the a strong possibility in (1) and a weak possibility in (2) that the bell may not even need to be pressed at all, let alone multiple times. – FumbleFingers Feb 7 '12 at 4:58

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