Why don't we contract "it is" in "If it is, then..."

Question

I wrote some instructions for a friend today, asking them to check something, and then act differently depending on the result:

It should be spinning when it's on. If it isn't then check the batteries. If it is then replace the frobulator.

I used the contraction "it's" for the first sentence to replace "it is".

I didn't use the contraction for the second because I chose to contract "is not" to "isn't". However I could have chosen the opposite: "If it's not check the batteries" would have been fine.

However, the third sentence doesn't sound right if I contract "it is". "If it's then replace the frobulator" seems incorrect.

Is there a rule that explains why "it is" shouldn't be contracted in this case, or am I wrong, and that contraction would be fine in that last sentence?

Answer 1

From The Cambridge Grammar of the English Language, p.1614:

Prepositions, auxiliaries, and infinitival to are stressed when they are the sole or final element in a phrase-level constituent, a PP or VP [preposition phrase or verb phrase].

Note that be is always an auxiliary verb. In your example, is is an auxiliary at the end of a verb phrase:

1a. If it is ___ then replace the frobulator.
1b. *If it's ___ then replace the frobulator.　　　←　ungrammatical

If we added the ellipted word spinning back in, it would no longer be at the end of the phrase, and it would be possible to contract it:

2a. If it is spinning then replace the frobulator.
2b. If it's spinning then replace the frobulator.

This is the same reason you can't reduce it is to it's at the end of a sentence. Is needs to bear stress, so it can't be reduced to clitic form (can't be contracted).

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In this answer, the * symbol means that a sentence is ungrammatical.