I'd like to know why the singular "ist" is the correct grammar, despite the two subjects connected with "und". Shouldn't you use the plural "sind" instead? Is it wrong to consider it in the same way as "A and B are" in English?
Wenn das Angebot groß und die Nachfrage klein ist, ...
Because these are acutally two sentences with ist:
Wenn das Angebot groß ist und die Nachfrage klein ist, ...
and you can omit repeated words. You need two, because they are two "equations": Angebot = groß, Nachfrage = klein.
If you equate them to the same thing, you use a two-part subject and sind:
Wenn das Angebot und die Nachfrage groß sind, ...
as you expected. It's the same in English: You say
If the supply is great and the demand is small, ...
If the supply is great and the demand small, ...
and not
If the supply are great and the demand small, ...
In your example, there is a single grammatical unit. "Wenn das Angebot groß und die Nachfrage klein" is a condition. This condition takes the form ist. You might rephrase it, "The condition of (das Angebot groß und die Nachfrage klein) is...
If you were talking about "A" und "B" separately, then the verb would be sind. Das Angebot und die Nachfrage" sind.
"The supply, and demand, are..." Here, I separated the two by removing the words groß and klein
