1) Understanding Complete Sentences
A complete sentence must include two essential features: a subject and a verb—and these two features must work together.
For example—
I write.
The above example is a complete sentence because it has a subject, I, and a verb, write—and they both work together. The simple inclusion of a subject and verb, however, does not guarantee that the two will work together.
For example—
I writing.
While this sentence still includes a subject and a verb, the verb—written in its present progressive form—cannot stand without a helping verb, such as am.
For example—
I am writing.
Similarly, the simple inclusion of a noun does not guarantee that noun will function as a subject.
For example—
Writing the book.
The above example includes the word book, but book cannot serve as the subject because it serves as a direct object. It does not act. It receives action. This means the sentence still needs a subject.
For example—
I am writing the book.
In the above example, the word I becomes the subject.
Keep in mind that length does not determine the completeness of a sentence. Many complete sentences require only one or two words. Meanwhile, fragments can be—and often are—dozens of words long.
Thoughts, Revised & Refined 