Interjections are the least important of all. They add liveliness to language, but they are not actual necessities. We could express all the thoughts that enter our minds without ever using an interjection.
35. A sentence may consist of but two words,—a noun or pronoun (the subject) and a verb (the predicate). Thus,—
Charles | swims.
Commonly, however, either the subject or the predicate, or both, will contain more than one word. Thus,—
Young Charles | swims slowly.
Here the complete subject (young Charles) consists of a noun (Charles) and an adjective (young), which describes Charles. The complete predicate consists of a verb (swims) and an adverb (slowly), which modifies swim by indicating how the action is performed. The subject noun (Charles) and the predicate verb (swims) are the chief words in the sentence, for neither could be omitted without destroying it. They form, so to speak, the frame or skeleton of the whole. Either of the two modifiers, the adjective or the adverb, or both, might be omitted, without destroying the sentence; for this would still exist as the expression of a thought (Charles swims), though the thought would be less definite and exact than it is when the modifiers are included.
36. The simple subject of a sentence is a noun or pronoun.
The simple predicate of a sentence is a verb or verb-phrase.
The simple subject, with such words as explain or complete its meaning, forms the complete subject.
The simple predicate, with such words as explain or complete its meaning, forms the complete predicate.