I've been learning german for a while and sometimes i come across something called weak nouns, which have to be declined like adjectives (eg. bekannte). I can't understand what those are, a little help?
I will try. Two things there: weak nouns (which tend to be masculine, and often end in E, or are professions) and adjectival nouns (which come from adjectives; i.e. if there is a word meaning well-known, and there is a similar noun meaning well-known person, then that is an adjectival noun and will have the same ending it had if it were an adjective. So, Bekannte is an adjectival noun. This is not exactly the same as a weak noun.
This page does a nicer job of explaining both weak nouns and adjectival nouns. http://www.vistawide.com/german/grammar/german_nouns03.htm