Consider, for example, how we can complete a frame sentence like "I saw ____." with different NPs.
|*the Fred||*the Netherland||the cat||the trash||the stone|
|*a Fred||*a Netherland||a cat||*a trash||a stone|
|*some Fred||*some Netherland||*some cat||some trash||some stone|
|*the Freds||the Netherlands||the cats||*the trashes||the stones|
The elements of this table flagged with asterisks are ungrammatical as completions for the given frame. In short, different nouns have different restrictions on what determiners they can take and on whether or not they can be made plural. This behavior is regular enough among groups of nouns that we can say that there are subtypes of nouns. We can explain the behavior of the nouns above by introducing two subdivisions: proper vs. common nouns, and count vs. non-count nouns.
 The frame sentence is deliberately brief to allow it to make sense with a wide variety of nouns, but because of this vagueness, some people don't see why some of the items in column 5 are not flagged as ungrammatical. If you're in that group, try expanding the frame sentence a bit to give yourself more context. For example, add ("in the courtyard").
 Some here should be read as the unstressed determiner with the meaning "an unspecified quantity," not the stressed word, which often means something like "a remarkable" (e.g., "She is some tennis player.")