I agree with Cherry,
when you grow up with a language you don't learn it in an abstract way by memorizing grammatic rules. you rather learn by trial and error. when kids say something wrong, they are corrected until they say it right. that's how they learn.
so when germans speak in their languages they just "feel" if something is said right or wrong.
but i must say, that in daily use of the language many germans don't stick too strictly to correct grammar. they mostly use the passive in a wrong way or use anglizised forms which are not correct but are tolerated because they are used so often.
So there actually IS a tolerance towards bending the grammatic rules a little bit. But that bending of the language mustnt corrupt the grammar in a way that will make what is said disunderstandable.
For example i can bend the german grammar using an anglizisim:
Ich wurde in 1983 geboren.
---> To put an "in" before the year is NOT conform with the German grammar. It's been taken over the last 10 years from english, possibly due to uncareful translations in the news. In the correct form the "in" would not be used.
Still if i say something like this, i'll still be understood because the is no ambigous understanding of the sentence.
But these are small changes. There is no tolerance for wrong grammar in numerus and casus and also correct word order because changes like these might make what is said disunderstandable.