Sure, rules of sentence structure exist. But if all writers follow them slavishly, we would not have sentence fragments like these in well-known books and essays:
- My birthday a year ago when he had twenty-five nights left to live. (Joan Didion, from The Year of Magical Thinking)
- To return to the convoy about to depart. (Viktor E. Frankl, translated by Ilse Lasch, from Man's Search for Meaning)
- But for what purpose? (Martin Luther King, Jr., from Letter from a Birmingham City Jail)
- Semitic? Semiotic? Jews and the science of signs? (Walker Percy, from "Why Are You a Catholic?" in Signposts in a Strange Land)
The point? Know the rules first and break them sparingly for dramatic effect.