Old Hebrew again. Casus constructus.
How do rivers נְהָר֣וֹת become נַהֲר֣וֹת in the constructus? I think I understand the reduced kamatz: The original kamatz had to be reduced, and it cannot be schwa because of the guttural, right? But I don't know where the patach under the nun comes from. (Schwa to patach? Or do they look back to the singular?)
Normally Lambdin is really good for self-study, I think I must be missing something obvious. Can you help?