"The boys are in Rome."
Translation:Pueri sunt Romae.
"Roma" is the name of a city. The names of towns, cities, and small islands take the locative case, which means they do not take a preposition.
All other nouns (with the exception of the words "domus", "humus", and "rus", which take the locative) take a preposition plus the ablative.
Latin is a little bit flexible. It's generally SOV (subject object verb), but "esse/to be" is a verb of state, not of action, so there is no object but rather a subject complement. Both SCV and SVC are valid. SVC is sometimes used to make it clear that we're saying "The boys are in Rome" and not "The boys in Rome are..."