Recursion allows individuals to embed phrases or sentences inside a larger sentence to generate infinite numbers of sentences. Since the number of lexical units in human languages is finite, productive rules like recursion exists in almost all languages. Chomsky (2010) mentioned, “the rules of the grammar must iterate in some manner to generate an infinite number of sentences, each with its specific sound, structure, and meaning.”
1. N’ → (D) N’ (PP+)
2. Adj’ -> (AdvP+) Adj
3. V' -> V' (PP+)
4. P’ -> P’ (PP+)
The generalized recursion rule is (X’ -> (WP+)X’), and there are also other possibilities for recursion in English.
For instance, in embedded complementizer phrases like “I think that she thinks that he thinks
that John runs slowly”, the complementizer “that” in embedded complementizer
(CP) is assumed to be there but can be optional, so it acts like an adjunct,
which is recursive and can be iterated to create an infinite amount of sentences.
Moreover, context is crucial to distinguish adjuncts from complement. In the NP "A
Chinese teacher", “Chinese” can be understood as the teacher's
nationality, which is an adjunct; or as a subject, “Chinese” would become a complement of the head “teacher”.
Note that the recursive rule generalized did not specify word order as word order parameters are language-specific, and every language has its own recursion rule. The above examples are based on English language. All in all, the recursion rule indicates there is an infinite number of sentences that humans can make.
Carnie, A. (2022). Syntax: A Generative Introduction (3rd Edition). Wiley-Blackwell.