So I'm working through a text book and wondering why I can't make the sentence "山田さんは、私の単一の友達." According to my colleague, you can, however, say "山田さんは私のある友達だ." Is it really something you can't grammatically say or does it just sound weird like saying "Yamada is my singular friend"?
Considering that you may be looking for something regarding
Yamada is my one friend
Two other options would be a better translation:
In this context, 山田さんは私のある友達だ is weird and does not make sense. I think it would be better to compare the difference between「単一」and「唯一」.
In English, we would be comparing Single 「単一」 and Only 「唯一」, which for itself may show that 「単一」 is not really appropriate in the given context.
「単一」is used to refer to the appearance of everything being the same and unified without exception, so it is used when there are many similar things that have no difference in appearance or performance, or they share similarities.
「唯一」is in a sense that there is only one in the world, that is unique.
Basically, when we have many of something/someone that are all the same/similar, we use「単一」, but if there is only one of many, so this is rare, unique, then we use 「唯一」