Learn Croatian: Trešnja je manja nego breskva. - A cherry is smaller than a peach.

When you use nego (than) for comparison, the word that follows it takes the same grammatical case as the thing being compared. Since trešnja is in the nominative as the subject of the sentence, breskva must also be in the nominative.

How would this sentence change if I used od instead of nego?

If you use od to mean 'than', the noun that follows it must take the genitive case. The sentence would become Trešnja je manja od breskve. Both structures are extremely common and completely interchangeable when comparing two nouns like this.

Are there situations where I am forced to use nego instead of od?

Yes. While you can use either nego or od when comparing nouns, you must use nego when comparing verbs, adverbs, or entire clauses. For example, you have to say Više volim spavati nego raditi (I prefer sleeping to working). You cannot use od in that context.

What is the base adjective for manja, and are there other common irregular comparatives I should remember?