Learn Russian: Раньше город назывался иначе.

Because иначе is an adverb meaning "differently" or "otherwise," and adverbs do not have cases. If you were stating the actual previous name of the city, you would use the Instrumental case, which is standard when using the past tense of называться. For example: Раньше город назывался Ленинградом (Previously the city was called Leningrad).

Why does the verb назывался have the -ся ending here?

The -ся ending makes the verb passive. The active verb называть means "to name" or "to call" something else. Because the city is the grammatical subject but is receiving the action of being named, we use the passive form называться (to be called).

Why do we use the imperfective aspect for назывался?

The imperfective aspect is used because it describes a continuous, ongoing state in the past (it "used to be called" this over a period of time). If you wanted to focus on the single completed action of giving the city a name, you would use a perfective construction like Город назвали... (They named the city...).

How does раньше translate to "used to"?

Russian doesn't have a direct equivalent to the English grammar structure "used to [verb]." Instead, you express this idea by combining a time adverb like раньше (earlier, previously) with an imperfective past tense verb. This naturally translates to "used to be" in English.