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✲ If you came tomorrow, [you were able to help with the flowers].
We can correct the apodosis by changing were to would be, but this change involves adding modal would.
(The Cambridge Grammar of the English Language, p.107)
CGEL only says were needs to be changed to would be. Can't could be be used instead of would be? If it can, what is the semantic difference?
There are two different ways of substituting could into this sentence.
If you substitute could for would you change the meaning. With would be able to help the would expresses contingency, but is otherwise solidly predictive: if the condition is in fact met, you definitely will be able to help. Could be able to, on the other hand, introduces another layer of uncertainty. In this context, could is functionally equivalent to might; so what is said is that if the condition is in fact, there is only a possibility you will be able to help.
Note however that can is equivalent to be able to. Consequently, you may substitute could not merely for would but for the entire verb construction would be able to:
If you came tomorrow you could help with the flowers.
In strict logic, this would be equivalent to the original emendation If you came tomorrow you would be able to help with the flowers. In practise, however, it would employed differently: depending on context it might indicate either
an offer of a tempting opportunity: "I know you love arranging flowers, so if you came tomorrow we could put that in your hands!"
or
a hopeful request for needed assistance: "I know you're very busy, but I'd be oh-so-grateful if you could find the time to give me a hand with the flowers."
Context, context, context!
