If you concern about your grade, make sure your exceed the expectations for online discussions and you can get up to 5 points of extra credit.
Now I understand bad grammar is more common than hot days in the Texas summer, and normally I try to give people a break on this type of thing. However, if you post the following in your syllabus, you have to be prepared to write grammatically correct English.
From the syllabus:
Edit your message before posting it. Numerous grammatical errors and spelling errors or a "stream of consciousness" approach makes reading your message tedious.
Yup.
(Inevitably, I've written something that's grammatically incorrect in my rant... pick on me, make fun of me, whatever. I'm not the one working on my PhD and teaching grad classes...)