Learn Hardcore Latin: Sine vi pax in urbe manet. - Without force, peace remains in the city.

Because sine is a preposition that takes the ablative case. The noun vis (force, violence) is irregular, and its ablative singular form is vi.

So:

vis = nominative singular
vi = ablative singular

That is why Latin says sine vi = without force / without violence.

Why is it in urbe and not in urbem?

Because in can take two different cases depending on the meaning:

in + ablative = in / on a place, showing location
in + accusative = into / onto a place, showing motion toward

Here the sentence describes where peace remains, not movement into the city, so Latin uses the ablative:

in urbe = in the city
in urbem = into the city

How do we know pax is the subject?

Pax is in the nominative case, which is the case normally used for the subject of the sentence. Also, the verb manet is third person singular, so it matches a singular subject like .