Questions & Answers about Pax in urbe servanda est.
They are:
- pax, pacis — peace
- urbs, urbis — city
- servo, servare, servavi, servatus — to preserve, keep, save
- sum, esse, fui — to be
So servanda comes from servo, and est comes from sum.
Because pax is the subject of the Latin sentence.
Latin is using a construction that literally means something like peace is to be preserved in the city. In that structure, the thing that must be preserved becomes the grammatical subject, so it stands in the nominative.
An English speaker may expect an object, as in we must preserve peace, but Latin often expresses this idea differently.