"We could only stay out until 11 P.M."
How long are you going to wait for an answer? Until 8pm? Until midnight? Until next Tuesday? No, "Until." is not a sentence.
'Is until tomorrow' is a fragment, because it has no real subject. You can make it a sentence: What Is until tomorrow?
It is an imperative sentence.
"Until" is the preposition in the sentence "They played until 11 o'clock." It shows the relationship between the playing and the specific time.
Example sentence - They would want to wait until they sky looked white too.
until you eat your vegetables
I play outside until it gets dark!
I will wait until you are done cleaning your room
The verb in the sentence is "wait."
Don't come back until 5 pm.
Waited
'Until the class begins' is not a complete sentence, and therefore it can not be classified. It is a clause, introduced by the conjunction 'until.' 'Until the class begins' is not a complete thought. We're still waiting for the rest of the sentence. Something will or will not happen, take place, be allowed, etc., 'until the class begins.' Without that something being stated, there is not a complete thought. It takes a complete thought to make a sentence. 'The class begins' is a complete sentence. It sounds like an announcement of some kind. It is perhaps a little awkward or stilted, but it is a complete sentence. It expresses a complete thought. Furthermore, it is a declarative sentence. It states a fact.