A sentence fragment is a string of words that may appear to be a complete sentence, but lacks either a subject or a verb, or even both, and thus fails to stand on its own as a full sentence. Sometimes, lengthily descriptive sentence fragments can deceivingly appear like sentences, but students need to ensure that there is a subject-verb relationship in the strings of words that they write, in order for it to be considered as a proper sentence.
Incorrect: Tirelessly toiling on his project, as he believes that hard work ultimately pays off.
Correct: Tirelessly toiling on his project, as he believes that hard work ultimately pays off, Philip had little sleep for a week, but felt relieved when he finally completed his work and submitted it to his teacher.
The above incorrect example is a sentence fragment, as it is merely descriptive, but is not a complete thought on its own – it needs the main subject to be present. In the correct version, the main subject “Philip”, and the verbs “had” and “felt”, are added in order to complete the fragment.