Well done! Getting that right shows that you already understand quite a lot about English syntax...because English syntax is simply how we sequence words in English to make sense.
If a friend said to you, 'The engineer', that would not make sense. To make sense, your friend would need to assert something about the engineer, e.g. 'The engineer built a robot.' That would make sense because it is a complete statement with a subject (The engineer) and an attached idea (built a robot). The attached idea is called the predicate.
The word 'predicate' comes from the Latin praedicatum, meaning 'to declare' or 'to proclaim'... so the predicate is simply what is declared (predicated) about the subject. When we attach a predicate to a subject, we have a complete thought - or in other words, a sentence.
The engineer
built a robot
The engineer built a robot