A preposition if it introduces a phrase, as in " until eleven o'clock "
A conjunction if it introduces a clause, as in " until the clock struck eleven "
An adverb.
it depends what word it is it could be any part of speech depending on the sentence
i am part of the CIA.
Some is an adjective.
'Don't' is part of the verb.
No, unless it is the first word in the sentence or part of a title.
I play outside until it gets dark!
I will wait until you are done cleaning your room
No, the word "juvie" is typically not capitalized in a sentence unless it is the first word or part of a title.
The word 'in' would be a preposition in the example sentence you gave.
type your answer
The term for knowing a word's part of speech in order to appropriately use it in a sentence is "word-usage."