

As far as I know, my Russian teacher never taught me why such things happen... And I've encountered so many cases other than these examples bearing same problems. Can somebody explain when and why this happens?


As far as I know, my Russian teacher never taught me why such things happen... And I've encountered so many cases other than these examples bearing same problems. Can somebody explain when and why this happens?
These are so called lesser cases (малые падежи). С боку на бок - отложительный падеж, or исходный падеж (ablative). Брал меду - отделительный падеж, sometimes also called количественно-отделительный падеж.
About lesser cases: What are the lesser known Russian cases?
с бокуis not местный падеж but rather отложительный падеж (аблатив), so I fixed it. As for your second question, you can say "набрал мед" instead of "набрал меда". However, if you say, for example, "он выпил воду", it means he drank all the offered water, while if you say "он выпил воды", it means he drank part of it. – user31264 Feb 10 '17 at 18:03