I go to an engineering school. That said, on every standardized test I've taken of the SAT form, I score high on the math...
... And higher on the English language portions of the test. The exception? The SAT essay. I suspect it's because my writing style is kind of concise, which isn't exactly what they're looking for, or something of that nature.
At said engineering school, everyone has to go through at least three English courses (more are optional, unless you need to make up ground). Those three are: Freshman Composition, Technical Communications I, and Technical Communications II. Posting here falls under at least the first two courses I mentioned.
I'm not going to neg rep somebody for making a spelling/grammar error. I've made enough of them myself. (Don't believe me? Check the posts I made in 2005 and 2006.) Its a learning process. Practice using the English language (or any other language, for that matter) properly, and your readability will improve over time.
As another note, go out and try to find spelling and grammar errors in a published book. It's interesting to try to find them. Sometimes, it's hard. Other times, it's easy. A couple of times, my mom (an English major) and I did it on the same book. I won't say who found more, or who found errors the other missed, but the errors were there.
And your spell check won't catch some of them. I've deliberately made one in this post. The spell check didn't catch it. Can you?