Learn Spanish (Latin America): La ciudad es totalmente diferente ahora.

To make an adverb, you usually add -mente to the feminine form of an adjective. The adjective here is total. Because total ends in a consonant, it doesn't have a separate feminine form (it is just total for both masculine and feminine nouns). So, you simply add -mente directly to the word to get totalmente.

Since the city is different 'now' (ahora), why use es instead of está?

This is a classic tricky situation! Even though the city's state has changed over time, being 'different' is considered a defining characteristic of what the city is like right now. We use ser (here as es) to describe core characteristics. If you wanted to emphasize the result of a change rather than a permanent trait, you would use estar with a word like cambiada (changed).

Why is it la ciudad instead of el ciudad?

In Spanish, nouns that end in -dad (like ciudad, universidad, and verdad) are almost always feminine. Because it is feminine, it uses the feminine word for 'the', which is la.

If la ciudad is feminine, why doesn't diferente change to match it?