It is a sentence because it contains both a subject and a verb.
"He will return." is a complete sentence; 'he' is the subject and 'will return' is the verb.
A sentence only requires a subject and verb - everything else is just details. "He wept" is a prime example of a viable sentence, and "he will return" is another.
Do you work? is a complete sentence; it is not a sentence fragment.
"She wave." is fragment but "She waves" is a sentence.
fragment
Fragment.
sentence fragment
It is a sentence fragment
Make each fragment into a complete sentence.
is a sentence fragment
"After the storm," is not a complete sentence so it is a fragment.
A sentence is a fragment when you do not have a noun/pronoun and a verb.