utro = morning, sutra = in the morning, zavtra = also in the morning but weirdly distorted for no obvious reason. There seems less etymological distortion in Serbian.
That's not an opinion, that's how basic Slavic morphology/grammar works.
For example, some Russians say "v ogorode" ("in the garden"), while others say "na ogorode". We don't say "v" somehow got "distorted"/morphed into "na". Some dialects simply choose to use a different, unrelated preposition in the same context.
