I always compare the differences in pronunciation of non-stressed «а», «о», «ы» with differences of pronunciation of English words «this», «these», and «be», «bee». These pairs sound completely equal for «untrained ear».
If some russian will say «у него было много работа» to another russian, the latter will immediately recognize an error in spite of similar sounding. It is nearly impossible for native russian to select incorrect grammatical case in such a simple case. The russian children can form and recognize grammatical cases of all words before they even hear these words. They can make mistake in forming of grammatical case of concrete word (for ex. say «у шкафы» instead of «у шкафа»), but never in selecting which case to use. Cases and prepositions conect words in a sentence together, into a single idea/message. So, for russians cases are so important, that they always hear which case do you use.
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I have a theory that cases and prepositions are direct representation of our brain logical structure. The concepts in our brain are connected into network with links of different types. The type of link is translated into grammatical case and/or preposition when you speak. When some elements (nodes and links) of the network are activated (for ex., «окно <= мыть <- мама», where arrows of different types represent links of different types) the brain trying to find the order of concepts which will follow all the links in «natural» order (the most natural order is temporal order), forming a sentence. That is why all languages have sentences, and most languages have grammatical cases.
Sometimes the concepts subnetwork you want to forward to your interlocutor's brain is complicated, so it can be expressed in different ways and in different number of sentences:
Code:
что делает> кого/что>
МАМА------------- >МЫТЬ===================>ВАЗА
| <кто делает ||| <что с этим делают /\
| ||| ||
| |||затем ||
| \|||/ ||
| что делает> \|/ кого/что> ||
------------->РАЗБИВАТЬ====================
<кто делает ||| <что с этим делают
|||
\|||/затем
\|/
[СЕЙЧАС]
(So, you see that «кого/что» is a type of link which directly translated into accusative case).
- Мама вазу мыла, затем её разбила.[/*:m:3vmfwapk]
- Мама разбила вазу, а перед этим её мыла.[/*:m:3vmfwapk]
- Вчера было такое! Представь, мама моет вазу, а потом её разбивает![/*:m:3vmfwapk]
- Мама вымыла вазу перед тем, как её разбить.[/*:m:3vmfwapk]
- Вначале мама мыла вазу. Затем она её разбила. Ну а потом уже мы с тобой встретились, и вот я тебе рассказываю это.[/*:m:3vmfwapk]
- Ваза была тщательно вымыта мамой, а затем ею же разбита.[/*:m:3vmfwapk]
In English the word order is used instead of grammatical cases. So, it is easy for foreign speaker to make a mistake. The foreigners are commonly paradised in our films in the way they using wrong cases. So, even 4-years-old child can laugh at them.