The phrase 'filius est rex' translates to 'the son is king' in English, illustrating a clear example of a predicate nominative construction in Latin. This phrase consists of a subject ('filius'), a linking verb ('est'), and a predicate nominative ('rex'), which renames or identifies the subject. Understanding how this structure works helps to clarify how nouns can function in different roles within a sentence, especially in linking sentences that describe relationships or attributes.