As an editor, I can say one reason so many people have such awful grammar is Microsoft Word. Whenever I run spelling and grammar check at the end of a job, I am appalled at what comes up. Most of the time I have something correct and Word asks me if I don't want to change it to X, which would be wrong! That goes for spelling, grammar, wording, sentence structure, capitalization, punctuation. Lesson: Word is not the last word.
If you care about something you’ve written, after you finish writing—and this is radical—read it aloud. It’s astounding how often something that sounds so right on paper sounds so wrong when spoken. Equally astounding is how often Microsoft Word deems something correct when it is so wrong, for example:
I no its knot write to say many of the things eye doo, butt when I right thumb, they seam to reed all write two I. Me even red them moor then too thymes.
I ran spelling and grammar check on the preceding and no errors came up. Eye sup hose it’s perfectly grate to pub lash these words as they err.