Translate the following sentence: Caesar barbaros trans flumen oppugnavit

First thing is to find the nominative, which in this case is Caesar. Then the main verb which is oppugnavit. You should identify the tense and person of the verb, here it is past tense and 3rd person singular, agreeing with Caesar. So already the sentence reads 'Caesar attacked'. Now to find the object of this verb if there is one, which is a word in the accusative with usual transitive verbs. Barbaros fits the criteria here, but also flumen, as that is accusative as well. The distinguishing factor here is 'trans', which takes the accusative (flumen). Trans means across, so 'Caesar attacked the river across the barbarians' doesn't make sense, or is less realistic than 'Caesar attacked the barbarians across the river'.