I'd like a nice Latin translation of an idea expressed in English.  It
is more important to capture the meaning of the phrase, and not the
precise wording.  Something poetic and rhythmic would be nice, but not
necessary.

The english is something like:

"If it isn't tested, it doesn't work" -OR-
"If it hasn't been verified, it won't function correctly" -OR-
"No testee, no workee" -OR-

You get the idea...

Nisi id probabitur, deficiet.


There are several ways to translate English conditionals into Latin.
Simple indicative forms can suffice.
 
Conditional sentences are expressed with the conjunction "si -- if".
"If he is doing it, he is wise."
Si id facit, prudens est.

Negative conditionals are expressed with the conjunction "nisi -- if not, lest".
"If he is not doing it, he is wise."
Nisi id facit, prudens est.

If the conditional is set in the future, then the future tense must be used.
"If he does it, he will be wise."
Si id faciet, prudens erit."

The negative:
"If he does not do it, he will be wise."
"Nisi id faciet, prudens erit."

The sentence:

If it is not tested, it will not work.

has the additional feature that it is passive rather than active in
the main clause.

Thus, the Latin would require the use of the negative "nisi" and the
passive voice in the main clause. To simplify:

"If it is not (will not be tested) tested, it will fail."

Nisi id probabitur, deficiet.


is, ea, id -- the third person pronoun in Latin, has three genders. Is
-- masculine, he. Ea -- feminine, she. Id -- neuter, it.

Id in the singular is declined thus:

id -- nom.
eius -- gen.
ei -- dat.
id -- acc.
eo -- abl.


probo, probare, probavi, probatus sum -- to test, to prove, -- first conjugation.

Future Passive Conjugation.
Singular

probabor -- I shall be proved.
probaberis -- You shall be proved.
probabitur -- It shall be proved.


deficio, deficere, defeci, defectus sum -- to fail -- third conjugation, "i" stem.

Future Active Conjugation.
Singular

deficiam -- I shall fail.
deficies -- You will fail.
deficiet -- It will fail.

Source:
Wheelock, Frederick M., Latin: An Introductory Course Based on Ancient
Authors, Third Edition, Barnes And Noble, NY, 1967.

hlabadie-ga

An excellent answer with far more detail than expected.

Thanks for the tip and the rating.

hlabadie-ga