Can someone explain genitive case for me??

p.s. instrumental case in plural -all genders end -ami , yami

instrumental case  ends  m.-om,em ,n.-om,em,f-oy,ey
with preposition C(russian) shows that i make something with someone(not alone)
я иду С братом-i go with my brother
without preposition shows that somebody makes something with helping something
я пишу ручкой= I write with pen

My opinion is this: you learn russian very well if you'll speak. First time you must only speak without any rules writing. In that case you'll understand all the sounding sistem of language from inside. Doing this you don't pay attention to the mistakes. Believe me there is many people russian who is mistaken with cases. It's not necessary making right the cases for the first time. Only speaking, the rules will be come after that. Good luck for you.

what are you talking about foof and silver fox? I know that this is important in Russian, you need not tell me. This is fr me to ask questions about it when i have problems with cases.