A major sentence is a full unit of language expressing a complete idea, containing a subject and a predicate that provides additional information about it. Most complete sentences are major and can be analyzed and broken down through diagramming of the structure and grammar used within them. A major sentence can be as simple as "The cat ran fast," which contains both a subject and predicate, or a more complicated expression such as "Walking slowly, the man staggered from his car and sat down quickly beside the fallen tree." By contrast, a minor sentence does not contain both a subject and predicate and cannot easily be analyzed through diagramming.
The primary distinction between a minor and major sentence is how completely it expresses an idea through inclusion of every element required. When most people refer to a "complete sentence," they are indicating one that is major. It consists of two elements: the subject, which is what the sentence is about, and the predicate that provides additional information. These two components create a complete idea in a major sentence, which can then be analyzed and broken down through sentence diagramming to determine the grammatical pieces used in its construction.
For example, in the short major sentence, "The cat is gray," there is a subject, which consists of the noun phrase "the cat." This noun phrase is made up of a determiner in the form of a definite article, "the" and a noun "cat." The rest of the major sentence, "is gray" is the predicate and provides a description of the subject. "Is" acts as a linking verb in this instance, and connects the subject to the subject compliment, which is the adjective "gray."
When this analysis can be performed, this is a major sentence, as it contains all necessary elements within it. A more complex example would be a sentence like, "The man threw a ball to his son, who caught it while falling." This still contains a subject, which is "the man" and everything else in it is the predicate. In this case, the predicate is quite a bit more complex and includes the dependent clause, which requires the rest of the sentence to make sense, "who caught it while falling."
A minor sentence, on the other hand, does not contain a subject and predicate, but still expresses a complete idea. These types of sentences are often found in common expressions. The repeated use of a certain phrase often imparts it with meaning that allows others to understand what is being said, even if the sentence itself does not inherently seem complete.
An example of this is a phrase like "The more, the merrier," in which there is no particular subject indicated and without context it is essentially meaningless. This type of minor sentence cannot be diagrammed, because it is lacking major elements that are not even hinted at by it. Greetings and other types of expressions are often minor, including "Hello," and "Good bye," as complete sentences.