I find I don't usually have a problem with the grammar side of the English language, it's more the spelling, and it's not even spelling words wrong, it's that I have a tendency to mix up my vowels in words, writing it how I say it out loud.
Like definitely, that's one that used to trip me up a lot. Used to spell it definately.
Another mistake I sometimes make myself when typing absent mindedly is awful vs aweful.
Awful is something terrible and Aweful is something full of awe.