In the framework of Noam Chomsky's Minimalist Program, items of the lexicon are of two types: with or without substantive content.
[1] Functional items can also be classified as closed class, that is, belonging to parts of speech that do not easily allow new members.
They are able to recognize functional items by hearing them frequently and also through phonological and distributional cues.
[9] Moreover, infants are able to distinguish between functional and lexical items based on phonological and acoustic cues.
[11] Children with specific language impairments have difficulties with a range of elements within functional categories.