Why don’t Ukrainian prepositions govern the nominative or vocative case?

Question

A sentence like “Чоловік з Украї́на дзвонив.” (A man from Ukraine called.) has a different meaning from “Чоловік дзвонив з Україні.” (A man called from Ukraine.). Yet the preposition (з) has the same form referring to the subject of the sentence (Чоловік), which can only be in the nominative, as it does in its locative form refering to the verb, and the difference in meaning can only be understood by word order. Or, how about the use of a preposition in the vocative case - Ді́вча́та з веснянками, об'єднайтеся! (Girls with freckles, unite!) to distinguish it from “Girls unite with freckles.”.

Perhaps I’m not grasping the proper application of prepositions to nouns. In a simple sentence like
“Чоловік дзвонив.”, Чоловік is in the nominative case and would require associated modifiers to be in the same case (e.g. Чоловік великий – The man is big, “big” in the nominative case, not Чоловік великого - “big” in the genitive case). The same with using “з Украї́на” with Украї́на in the nominative case.
Could it be that a preposition is assigned a case and that case is used whether the noun it refers to is the subject, object, or any other part of sentence?

Answer 1

A man who called

Let me fix you, correct:

• Чоловік з України дзвонив • A man from Ukraine called
• Чоловік дзвонив з України • A man called from Ukraine

Yes, чоловік can only be in the Nominative if it’s the Subject.

But not in all the situation verbs can just change a related word, but prepositions — yes. Need just learn pairs of verbs with prepositions with their cases, also you can see the last paragraph here about a few notes with з. In this case з is a preposition as from and changes the related word into the Genitive case.

And yes, the difference in correct meaning might be understood by word order. But remember:

• Sometimes somebody can play with words' meaning;
• In a real speech, you can understand by pauses or intonation, context.

The girls and freckles

That is why the comma is an important part here, especially in the Vocative case:

• Girls with freckles, unite! • Дівчата з веснянками, обʼєднайтеся!
• Girls unite with freckles! • Дівчата, з веснянками обʼєднайтеся!

In a real speech, again, you can understand by pauses or intonation. Notice that з as with change the related word into the Instrumental case. Or in the second sentence just move the part з веснянками to the end. And just for vocabulary, веснянка has synonym ластовиння.

The big man

Just to notice, in almost all the standard cases, adjectives are written before a noun, just as in English. That is why:

• Великий чоловік • The big man

Here is an adjective for the Subject noun, so in the Nominative case. Of course, you can say the adjective after the noun, but it can be a little strange, poetic, archaic etc. So, but in your sentence:

• The man is big • Чоловік є великим

Big is not a Grammatical modifier (означення) for the Subject but close for (if I did not make a mistake) the Predicate and in the Instrumental case.

And just for comparison:

• Чоловік великого розміру • The man of big size

Here is the Genitive case.

The prepositions з

With the Genitive case:

• A movement from the inside, from a certain environment, from the surface: зі школи • from school, з поля • from the field, з хмари • from the cloud, з роботи • from work;
• A reason, basis of action: з жарту • from a joke, з нудьги • from boredom, з холоду • from cold, з дозволу • with permission;
• A material, implements: з дерева • (by) from a tree, з чашки • from a cup, з лука • with an onion;
• A sign for the branch: (підручник) з географії • (textbook) in geography, (фахівець) з медицини • (specialist) in medicine;
• The object of negative attitude: (глузувати, насміхатися, знущатися, кепкувати, глумитися) з когось • (ridicule, laugh, mock, cheat, mock) from someone.

In conjunction with the Accusative case of the noun, the preposition means "with an approximation, comparison": з десяток • about ten, з годину • about an hour, з діжку • about a barrel, з кулак • like a fist.

In conjunction with the Instrumental case of the noun, the preposition "with" indicates the interconnection of objects, the accompanying circumstances: з братом • with a brother, з людьми • with people, з шумом • with noise, з плугами • with plows, з проханням • with a request.