Could you help me to answer this question which is very problematic for me.Thanks in advance:)

21.ONE LEXICAL ITEM WILL OFTEN CORRESPOND TO A WORD (CAT) OR A PHRASE (A HARD NUT) WHILE ONE PHRASE NEED NOT CORRESPOND TO A LEXICAL ITEM. EXPLAIN USING EXAMPLES AND NOTIONS SUCH AS 'CONSTITUENTS', 'PRODUCTIVITY', 'OPAQUENESS'.