Grammar doesn't indicate how trustworthy you are, and really doesn't have a huge factor I believe, In becoming a GM.
I mean, try lurking IRC when the GM's are on xD
Seriously though; Just do what Flamecake said; Help people out as much as possible, Demonstraight your knowledge and such.
Then possibly when it comes time for the GM's to hire another one, They might pick you.
Although I wouldn't get too hopefull.