Per sepem ad agrum non itur, sed per portam.

Yes — itur comes from ire.

What is unusual is that this is the impersonal passive of ire. In Latin, some intransitive verbs can form an impersonal passive meaning:

• one goes
• people go
• there is going

So non itur means something like:

• one does not go
• people do not go
• it is not customary to go

A very literal English version like it is gone would be misleading here. The real sense is impersonal: people do not go that way.

Because itur is impersonal.

Latin often leaves out a subject when the verb form already makes the meaning clear. With an impersonal verb like itur, there is no specific person such as he, she, or they. The idea is general:

• one goes
• people go

So the sentence is not about a particular person. It states a general rule or habit.

All three are accusative singular.

That is because they are each governed by a preposition:

• per sepem = through the fence/hedge
• ad agrum = to the field
• per portam = through the gate

These prepositions take the accusative:

• per
  • accusative
• ad
  • accusative

So the forms are:

• sepes, sepis → sepem
• ager, agri → agrum
• porta, portae → portam

Because per means through, by way of, or along, and it takes the accusative.

Here it describes the route taken:

• per sepem = through/by way of the fence or hedge
• per portam = through the gate

The contrast is important: you do not go by forcing a way through the fence; you go through the gate instead.

Ad agrum means to the field, focusing on movement toward a destination.

Latin often distinguishes:

• ad
  • accusative = to, toward
• in
  • accusative = into

So:

• ad agrum = to the field / toward the field
• in agrum = into the field

In many contexts, English might use either depending on the situation, but Latin is being more precise about the destination. This sentence presents the field as the place one is heading to.

Sed means but.

It sets up a contrast:

• non itur ... sed ...
• one does not go ... but ...

So the sentence contrasts two routes:

• not through the fence
• but through the gate

It is a very common Latin way to correct or replace one idea with another.

Because non normally negates the word or idea that follows, and here it negates the verb:

• non itur = one does not go

That is the simplest and most natural placement.

Latin word order is flexible, but this placement makes the negation very clear.

Yes, a little.

The sentence is:

• Per sepem ad agrum non itur, sed per portam.

Latin often puts important or contrasting phrases in noticeable positions. Here the two route phrases are highlighted:

• Per sepem ...
• sed per portam

That makes the contrast very clear.

A more mechanical English-like order might be something like:

• Non itur ad agrum per sepem, sed per portam.

But the actual Latin order sounds more pointed and rhetorical.

It helps to divide it like this:

• Per sepem = route
• ad agrum = destination
• non itur = one does not go
• sed per portam = but through the gate

So the full sense is:

• route not used: through the fence
• destination: to the field
• verb: one does not go
• corrected route: but through the gate

Latin often leaves out repeated material. After sed, the verb itur and the destination ad agrum are understood:

• sed per portam = but through the gate [one goes to the field]

Yes. Sepes can mean hedge, fence, or an enclosing barrier.

Which English word is best depends on context. In many passages, hedge may be more natural; in others, fence works well. The important idea is that it is a boundary, and the proper entrance is the gate.

So the contrast is:

• not through the boundary itself
• but through the proper opening in it

Because Latin often omits words that are easy to understand from the previous clause.

After sed, the listener automatically understands something like:

• sed per portam itur
• or even sed per portam ad agrum itur

This omission is very common and natural. English does it too:

• You don’t go through the fence, but through the gate.

We do not have to repeat you go in the second part, because it is already understood.