When is a sentence complete?

Answer 1

A sentence is said to be complete when it contains a subject, a verb, and makes complete sense.
A sentence is a grammatical unit that contains one or more words that expresses an independent statement, question, request or command.

A complete sentence always has a subject and a verb.

There are some sentences that can be just one word. An imperative sentence can have an implied subject, for example: 'Stop!' or 'Look!'; the subject is implied: 'You stop!' or 'You look!'

In print or writing, a sentence typically begins with a capital letter and ends with the appropriate punctuation. It is the largest grammatical unit in language communicating a complete thought.
Many teachers believe that a complete sentence must always contain a verb. Sometimes the requirement is for a 'finite verb'.

I don't believe that this is true - but if I need to keep a teacher quiet I am usually willing to give this answer.
Every complete English sentence must include a subject and a predicate, which is to say, the sentence is about something, which is the subject, and the sentence also describes some action or condition of that subject, which description is the predicate. However, English can be tricky. Hence, the subject and/or the predicate can be implied rather than actually stated. So you can have a sentence consisting of a single word. Consider this discussion: "Do you want to have dinner? Yes." So the sentence "Yes" consists of one word. Does it contain a subject and a predicate? Yes, it does, because most of the sentence is implied, rather than stated. If we were to make the entire sentence explicit rather than implied, what it is really saying is "Yes, I would like to have dinner."