A sentence must have a subject (noun) and a verb (action). However, when we speak we don't always use complete sentences. So there are sentences that are made up of just one word followed by a punctuation mark. This is allowable because in one-word sentences either the noun or the verb is implied. So what are examples of these one word sentences? Here are some examples.
Goodbye! Hello! Hi! Stop. Go!
So in 'Stop." and 'Go!' the subject 'You' is implied. Interestingly the word 'Goodbye' is both an interjection and a noun in itself. As you can see, we usually see these one-word sentences in commands or greetings. You may argue that questions can also have one-word sentences and this is also allowed in spoken language. Here are some examples.
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