Could somebody specializes in SYNTAX help me explain the term subphrasal constituent? What is subphrasal constituent? How to define it?
A constituent is subphrasal (lit. ‘below the phrase’) if it alone does not qualify as a phrase. Consider a noun phrase such as the house. Together the two words the and house form a phrase and hence the two together create a phrasal constituent. Each of the two words alone, however, does not qualify as a phrase and is hence a subphrasal constituent.
We can examine a larger example. Consider the following phrase structure tree taken from the Wikipedia article on the constituent unit:
This tree indicates the presence of three phrasal constituents: NP, VP, and VP (the subject noun Drunks can also be viewed as an NP, although it is not shown as one in the tree). The analysis hence assumes three phrasal constituents. The entire tree, however, posits the presence of nine constituents altogether. There are therefore at least five constituents that are subphrasal (ignoring the status of Drunks). The individual words the, customers, put, off, and could are all subphrasal in the relevant sense. They are all parts of phrases, but each of them alone does not qualify as a phrase.
I can also provide an interesting side note. My particular specialization in linguistics is dependency grammar (DG). In DG, the distinction between phrasal and subphrasal constituents disappears. All constituents are in fact phrasal in the relevant sense. Examine the next DG tree of the same sentence, which is also taken from the Wikipedia article on the constituent unit:
This tree includes just five constituents (= complete subtrees), and each of these constituents is best construed as phrasal. The individual words could, put, and customers cannot be construed as constituents because each of them dominates other words and therefore does not alone qualify as a complete subtree.
The point, then, is that the distinction between phrasal and subphrasal constituents is particularly relevant when one has the distinction between dependency syntax and phrase structure syntax in mind.