I HATE, hate, hate the "would of, could of, should of" error. If people would read, they'd realize how wrong this is.
CallGirl, the "effect/affect" issue affects a lot of people! But, actually, grammar guides are wrong when they tell you that affect is always a verb and effect always a noun. (But I won't even get into that--LOL!) I think the best way to learn is to just keep looking it up when you're not sure which one to use. Soon it will be second nature.
A lot of people make the everyday error of using "alot," when that's not even a word!
I've been a professional writer for more than 30 years, and I make mistakes. I use the dictionary, thesaurus, and style guide every day. I don't understand why people don't look anything up. There's no excuse these days. Not when looking things up is easier than ever. I guess most people just don't care and/or don't know that they don't know.
Sillysister, I adore commas!
I try to use them correctly and judiciously, but I admit to continuing to use them, for example, after short introductory clauses. Which isn't wrong, but the custom is outdated. I'm trying to eliminate them unless the sentence makes more sense with than without. But I find that people don't use them enough these days, or stick them where they don't belong. I'm not sure what things schools are teaching, but proper comma usage must not be one of them!
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.