A group of words must meet the following three criteria to form a complete sentence. If it does not meet all three, it is considered a fragment.
A sentence must have a subject.
A sentence must have a verb, not just a verbal. A verbal (such as the participle singing) needs a helping verb in order to function as a sentence’s verb.
VERBAL
She singing.
VERB
She is singing.
Unless it is a question, a sentence must have at least one clause that does not begin with a subordinating conjunction such as because, if, that, or when.