It is a sentence. It is a declarative sentence also because it's giving a demand.
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.
No, it's a complete, but very short, sentence.
"It's here" is another way of saying "It is here". The word "it" is the subject, "is" is the verb, and "here" is an adverb modifying "is".
This sentence is indeed a sentence fragment because it fails to have a subject.
fragment
It is an fragment.
Sentence
Fragment.
A fragment sentence is a partial sentence. It does not have everything it needs to be a full sentence. For example: "The dog" is a fragment.
sentences
You should not capitalize a sentence fragment following a colon.
My finger was bleeding, there was a fragment of glass stuck in it.
D Here is. This is a fragment sentence because there is no subject in the clause.
sentence fragment
You can't really have a predicate fragment. Here is a predicate sentence: Judy (runs to the beach every day). (Runs to the beach every day) is the predicate fragment.
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.