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?