It could be either. 'Sit!' as an imperative form of the verb to sit (an instruction given to a dog, for example) is a sentence in its own right. Sit can also be just one word in a sentence, for example 'I asked you not to sit there.' In that case it would be a fragment.
It is a sentence. It is a declarative sentence also because it's giving a demand.
"Sit down and hear" is a sentence fragment, as it lacks a subject and a main verb. To make it a complete sentence, you could add a subject and a verb, such as "Please sit down and hear the presentation."
Do you work? is a complete sentence; it is not a sentence fragment.
fragment
"She wave." is fragment but "She waves" is a sentence.
Fragment.
sentence fragment
It is a sentence fragment
Make each fragment into a complete sentence.
"After the storm," is not a complete sentence so it is a fragment.
is a sentence fragment
A sentence is a fragment when you do not have a noun/pronoun and a verb.