Why is the pronunciation of “Ukrainian” in “Я розмовляю українською.” (I speak Ukrainian.) different from the pronunciation of “Ukrainian” in “Я розумію українську.” (I understand Ukrainian)?
It is because of the Adjective cases with Hard Declension (
The first sentence has verb розмовляти (to speak) which governs the Instrumental case (with/using what?) — українською.
The second sentence has verb розуміти (to understand) which governs the Accusative case (a direct object of action) — українську.
The Wikipedia article contains further introductory information about the Adjective cases in the Ukrainian language.
Just in case, adjective українська is feminine because it is for the noun мова (language) which is feminine and optional in speech. And, yes, nouns also have the same Cases (more information in other paragraphs) with their own declensions.