an interjection can form a complete sentence (that is, expressing a thought without need of a subject and verb): ouch, wow. etc

Major and minor sentences
A major sentence is a regular sentence; it has a subject and a predicate. For example: "I have a ball." In this sentence one can change the persons: "We have a ball." However, a minor sentence is an irregular type of sentence. It does not contain a finite verb. For example, "Mary!" "No" "Coffee." etc. Other examples of minor sentences are headings (e.g. the heading of this entry), stereotyped expressions ("Hello!"), emotional expressions ("Wow!"), proverbs, etc. This can also include nominal sentences like "The more, the merrier". These do not contain verbs in order to intensify the meaning around the nouns and are normally found in poetry and catchphrases.[3]
Sentences that comprise a single word are called word sentences, and the words themselves sentence words.[4]

A pro-sentence is a function word or expression that substitutes for a whole sentence whose content is recoverable from the context. A pro-sentence is a kind of pro-form and is therefore anaphoric.
In English, yes, no, okay and amen are common pro-sentences. In response to the question "Does Mars have two moons?", the sentence "Yes" can be understood to abbreviate "Mars has two moons."
Pro-sentences are sometimes seen as grammatical interjections, since they are capable of very limited syntactical relations. But they can also be classified as a distinct part of speech, given that (other) interjections have meanings of their own and are often described as expressions of feelings or emotions.

fine is a minor sentence and a pro sentence

JJMBallantyne is correct!!!

http://www.cws.illinois.edu/workshop/writers/subjects/

Additional Facts About Subjects
The "Understood You"
Sometimes, as in the case of imperative sentences (see verb mood ), the subject does not actually appear in the sentence. At such times the invisible subject is called the "understood you":

(You) Rent The Last of the Mohicans from the video store.

therefore:

Eat.
Die.
Talk
Sit.

when used imperatively are complete sentences.