"They are cleaning the big veranda."
Translation:He siivoavat isoa verantaa.
could you translate you reply into regular person english? i have read several articles on the cases and still cannot figure out what these cases mean. i have gleaned from them that in many of them they don't really exist in my native (and only) language so the concept is not being absorbed. my friend who is a native speaker cannot even explain them to me. it is much like when i can tell him that he is using english incorrectly but i cannot give him formal grammatical reasons why, i just know when a sentence is correct because i have spoken english for nearly six decades.
It's unnecessarily difficult to analyse a language without figuring out the terms that are used to describe it, and it's just as difficult for me to describe it without them. Think of them as tools to build your understanding of the language. You're not going to get far building it with your bare hands.
Nominative and partitive are among the 15 cases of Finnish. Each case assigns some sort of role to the words that they modify, and in doing so they establish syntactic links that would be established in English via word order and prepositions. Because these links are established via inflection, word order isn't as important in Finnish. Changing the word order in English changes meaning. The usual order is subject-verb-object-manner-place-time. If you switch the order of the subject and the object in the sentence "the dog chases the cat" by turning it into "the cat chases the dog", it also swaps their roles. This is not how it works in Finnish because of the cases.
Nominative case usually assigns the subject. Partitive has several roles. Among those roles is assigning the object of an irresultative and/or unfinished action. The Finnish translation of the example sentence I just used is "Koira jahtaa kissaa". If you swap the subject and object by turning it into "Kissaa jahtaa koira", the roles remain the same because the syntactic links are maintained by the cases. "Koira" has nominative case, which doesn't have an ending. You could consider it the most neutral case. "Kissaa" has partitive case, which has the -a ending. To swap the roles, you'd have to swap the cases, not the word order.
To circle back to this particular sentence, the adjective is part of the object, along with the noun. It's an object because it's acted upon by the verb. That verb is in a tense that indicates an ongoing action, so the action is unfinished. Because it's unfinished, the object of that action requires partitive case. Otherwise it would require accusative case. I strongly recommend looking up more detailed information about these cases and all of the others as well. Since they are such an important part of the language, figuring out their functions is pretty much essential for a non-native learner.