As we all know in schools, the problem with rules is they are dogged by exceptions. This applies to grammar as much as to anything else. However hard we try, our actions can so easily contradict our words.
Suppose you're reminding your class about the importance of verbs.
You've just told them to make sure that every sentence they write contains one. Then someone asks: "Does that mean that every sentence has a verb?"
And you answer: "Yes" in a sentence which is apparently verbless.
If every sentence needs a verb, how come the little word "yes" can act as a complete sentence on its own? Talk together about the three possibilities:
* "Yes." is not a complete sentence.
* "Yes." somehow counts as a verb.
* The rule about sentences needing a verb is wrong - they need either a verb or a word like "yes."
The first solution, that "Yes." isn't a sentence, certainly answers the problem. But it creates two new problems, one of which is punctuation.
If "Yes." is not a sentence, it doesn't deserve a full stop and a capital letter; but surely that flies in the face of normal usage by respectable writers.
The other weakness of the first solution lies in the meaning: "yes" has exactly the kind of meaning we expect from sentences. For example, if you ask: "Is it raining?" and we answer: "Yes.", then our answer means exactly the same as the sentence: "It is raining."
That's what "yes" is for - it's a word which stands for a preceding sentence in just the same way that a pronoun stands for a noun.
Solution 2: "yes" is a verb. The best that can be said for this solution is that it shows imagination.
"Yes" has none of the grammatical characteristics of verbs - for example, it has no tense (past or present), nor can it take a subject (compare "it is" with the impossible "it yes").
The verb "to yes" really won't impress a key stage 3 class.
If the solution is neither of the first two, it must be the third. "Yes."
must be a special kind of sentence that doesn't need a verb. It is - it's what grammarians call a "minor sentence".
The class will enjoy supplying further examples of minor sentences such as "Certainly." and "Maybe." plus numerous examples from the world of advertising. (And how about sentences like this?) Although minor sentences are classed as "minor", they are quite a common part of the way we speak and write. Are they worth adding to the writer's toolkit? Yes, of course.