instrumental case is primarily used to express the instrument/means of doing/having something with(c/co), i drink tea with sugar/milk- я пью чай с сахаром/ с молоком. also used in expressions like С новым годом/С днём рождения.

http://en.wikibooks.org/wiki/Russian/Grammar/Instrumental_case Like the other cases, there are prepositions which can place whatever comes after them in the instrumental case. There are six in total: С(о), над, под, за, перед, and между. C(o) + inst means 'with'. This may be confusing, as the primary meaning of the instrumental case on its own is also 'with'. There are two concepts that are both conveyed by the English word 'with', but are conveyed separately in Russian. 'With', as in 'by means of' ('He built it with his hands') is conveyed by the instrumental case on its own ('his hands' in their inst. form). 'With', as in 'accompanied with' ('He built it with her') is conveyed by c(o) followed by the instrumental case ('with her' is c + inst.). It can also be used to denote the manner in which something is done ('He built it with easy'). In both of these uses, c(o) inst is the antonym of без + gen ('without'). Над means 'over' or 'above', and под means 'under' or 'beneath'. Под + inst means 'under' is there is no motion involved - it there's motion, use под + acc instead ('The book is under the bed' - no movement, 'The book slid under the bed' - movement). Под + inst can also mean 'physically near', and под + acc can mean 'temporally near' or 'near in time' ('They live near the city' - physically near). мной =мною, лично я всегда говорю МНОЙ

это ТВОРИТЕЛЬНЫЙ падеж

а можно сказать mak? - пойдем со мной/ со мною.

>> Why does я have the forms мною и мной? How are these two forms used? Not only я has these forms: тобой - тобою There is no difference between мной и мною. Both are used in everyday informal conversation, but, for formal conversation, please, choose 'мной'. >>Как сказать "instrumental case" по-русски? Творительный падеж