First of all, I'd like to introduce myself in Finnish:
Moi! Minun nimeni on Iván. Minä olen 16 vuotta vanha. Minä assun Barcelonassa. Minä opiskelen suomea kotona. Minä puhun espanjaa, katalaanii, englantia, raskaa ja saksaa. Minä haluan oppia suomea, koska minusta Suomi on paras maa. Minä haluan asua Suomessa.
Secondly, I think I do know when to use partitive, plural with -t or genitive.
Partitive - The action hasn't finished yet. Something you're still doing (the present continous) When talking about things you can't count. Negative forms. After number bigger than 1.
Plural (-t) - When you specify the number of things you've done. When you ask about something (all the thing) which is not countable. When using the imperative but when we specify the number of the things. Something at all.
Genitive - When something has been finished and completed. When you're doing something but you know you will complete it. It's used with "varmasti". When you're buying and know that you will buy just one thing. When you know just "one thing" of a group.