Most of the problem of understanding language hinges on the knowledge of the mechanics of that language. All languages have structure, and an implicit knowledge of that structure is essential to language comprehension.
Linguistic Knowledge is the synthesis of three more basic cognitive elements -- phonology, semantics, and syntax. Linguistic knowledge is more than the sum of its parts, but it does not lend itself to explicit assessment. A child may have a grasp on the more basic cognitive elements, but still have trouble blending these elements together into a stable linguistic structure. For example, if a child appears to have a grasp of the more basic cognitive elements, but is still having difficulty expressing herself or understanding others, it is likely that the child has not yet managed to synthesize those elements. Similarly, children who have limited appreciation for genre or different voices in stories may be having difficulty synthesizing the more basic linguistic elements together. Or, children who communicate well informally, but who have difficulty with more formal linguistic structures (which are often found in classroom settings) may be having difficulty applying linguistic knowledge. An assessment of linguistic knowledge is less straightforward than other cognitive elements, and is often revealed through elimination of other possibilities.
Teachers should be careful not to confuse dialect differences with a lack of linguistic knowledge. Teachers should also know that, especially when considering a child's appreciation of genre, voice, and formal linguistic forms, children who primarily speak languages other than English may have more advanced linguistic knowledge in their native language than in English.