Basic grammar questions concerning prep case and adj/noun agreement

There are two sentences in my textbook that have me confused:

1) While summing up the chapter, a sentence in the book read «Мы говорили о падежах, числительных, предлогах . . .

These are all nouns ("cases, numerals, prepositions" respectively) but why does числительных have the prepositional case ending for plural adjectives «ых»?

2) Another simple question here: Whenever I see questions asking "on which day?" I see it printed as «в какой день?» But if день is a masculine noun shouldn't it be «в каком день?»

Any help is appreciated.

1) Some nouns look and behave like adjectives, and числительное is one of them.
So you will need to decline these words as adjectives, for example:
-Пожарный
-Гостиная
-Числительное https://ru.wiktionary.org/wiki/%D1%8...BD%D0%BE%D0%B5


2) For some reason, we don't say "в каком дне"
"В какой день" is acceptable but rare.
I think it's a kinda set phrase, and usually you will see just "когда" instead.
There are more "exceptions", of course (and I suspect, that nouns are here in fact in Accusative):
-в первый день весны
-сын пошёл в первый класс
-в этот раз
-в кои-то веки
-в один присест
...
More examples, all escaping "в дне" thing:
- На какой день назначено совещание?
- На какую дату ты взял билеты?
- Могу устроить. Так вам на когда?
- Когда премьера?

Another tricky word you will encounter soon --
"в" + "час" = "в часу"
- в котором часу она проснулась?
But suddenly, speaking figuratively (not about specific point in time, but of some more prolonged "times"), we are again on the light side:
- в час опасности, в недобрый час, ...

Sorry on both points :"":

Very helpful. Спасибо!

Quote:

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Originally Posted by lenssideways8

Another simple question here: Whenever I see questions asking "on which day?" I see it printed as «в какой день?» But if день is a masculine noun shouldn't it be «в каком день?»

Any help is appreciated.

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This is the Accusative case.

We use different structures for different periods of time in Russian, there is no particular logic in it. (Basically it is the same in English - they say 'On Monday', but 'in Summer', but 'at night' - no logic really)

And, of course, questions usually have the same structure as answers. This is the basic rule for Russian, English and most other languages I believe.

So what we have in Russian.

Dates
Genitive case with no preposition: Восьмого октября, пятого марта. And the question is alson in Genitive: Какого числа?

Months
В + Prepositional case: В сентябре, в октябре. So the question is also in Prepositional: В каком месяце?

Years

В + Prepositional case: В 1987 году, в прошлом году. So the question is also in Prepositional: В каком году?

Days of the week
В + Accusative: В понедельник, в среду. So the question is also in Accusative: В какой день (недели)?

Basically this is the answer to your question. As you may see, nothing really complicated, just something you need to learn and remember.

However, sometimes, the questions do not match the structure of answers if we talk about Seasons and Parts of 24Hours
We use Instrumental case for them: ночью, утром, летом, зимой. But the questions will be В + Accusative: В какое время года? В какое время суток?