Introduction to Ling ...

Competence refers to a person's internalised grammar (knowledge) of his language. This means a native speaker's ability to produce and understand sentences, including sentences they have never heard before. It also includes a person's knowledge of what are and what are not sentences of a particular language.

A constituent is a group of words or morphemes with closer relationships between one another than between the elements of the other groups or constituents within the same sentence.