The grammatical name given to the expression "the first wife" is a noun phrase. A noun phrase is a group of words that functions as a noun in a sentence. It can consist of a single word (e.g., "cat") or multiple words (e.g., "the black cat sitting on the mat"). In this case, "the first wife" is a noun phrase where "the" acts as the definite article and "first" modifies the noun "wife". Noun phrases are used to name people, places, things, ideas, or concepts in a sentence.
