Definition: A sentence fragment is a group of words that is punctuated as a sentence but lacks one or more of the following: (1) a complete subject, (2) a complete verb, or (3) a complete idea.
Example: When I was merely a child.
Correction: When I was merely a child, I experienced a grave illness.
Example: The man running frantically through the park.
Correction: I observed the man running frantically through the park.