Learn Russian: Раньше Иван был толстый, а сейчас он худой. - Earlier Ivan was fat, but now he is thin.

Yes! раньше is the comparative form of the adverb рано (early). While it literally means 'earlier,' it is the most common way to say 'in the past' or 'previously' in Russian. Even though the adjectives толстый and худой aren't in the comparative form, the sentence still uses a comparative adverb to contrast two periods of time.

Why use а for 'but' instead of но?

We use а because we are simply contrasting two parallel facts (Ivan in the past versus Ivan now). The conjunction но is used when there is a contradiction or an unexpected result, such as 'Ivan is fat, but he eats very little.' For straightforward comparisons of states or people, а is the right word.

I learned that adjectives after был should be in the Instrumental case. Why is толстый in the Nominative case here?

In modern Russian, both Nominative and Instrumental are used after the past tense verb был. The Nominative (толстый) often sounds more natural in casual, everyday speech, or when describing a simple fact. The Instrumental (толстым) usually emphasizes that a state was temporary. Both would be grammatically correct in this sentence.

Why does the first half have a verb (был) but the second half does not?