Sepes hortum a via dividit.

Because hortum is the direct object of dividit. The noun hortus is a second-declension masculine noun, and its accusative singular form is hortum. So:

• hortus = subject form, nominative
• hortum = object form, accusative

Latin shows a word's job in the sentence by its case ending, not mainly by word order.

A means from here, and it introduces the thing from which the garden is separated. With verbs like dividit in this kind of sentence, Latin often uses:

thing being separated + a/ab + source of separation

So hortum a via dividit means that the garden is being separated from the road.

After a/ab, Latin uses the ablative, so via here is ablative singular.

This can be confusing because the first-declension ablative singular is really viā with a long ā, but many texts do not write macrons. So without macrons:

• nominative: via
• ablative: via

They look the same in ordinary spelling, even though they are different cases.

Dividit is the present active indicative, third person singular, of dividere.

That means:

• present = it is happening now, or is stated as a general fact
• active = the subject performs the action
• third person singular = he/she/it divides

Since the subject is sepes, the sense is the fence separates.

Yes, this word order is perfectly normal. Latin often places the verb at the end, as in dividit here.

Because Latin uses case endings, the word order is more flexible than in English. The sentence could be rearranged in other ways and still keep the same basic meaning, as long as the case endings stay the same. This order is a straightforward one:

subject + object + prepositional phrase + verb