Some of the grammar rules that were drilled into my head aren’t rules at all. That is, grammar nazis (a group in which I do not include myself) have emphasized style preferences to the point that people now think they are rules. In fact, they are not. I would use them sparingly, simply so people don’t think you’re stupid, but use them I do.
In writing, please remember people don’t speak the Queen’s English (except for, you know, the Queen). They talk how they talk. So ignore these faux rules in dialogue whenever you see fit. (As an aside, why do we add the “ue” in dialogue, when dialog is fine? Grammar nazis, that’s why. They make us overthink everything.)
1. Do Not End a Sentence With A Preposition – The problem here isn’t the preposition, it’s unneeded words. (Say what?) If you need a preposition for the sentence to make sense, leave it in.
Example (Holding her nose): “My god, what did you roll in?” Here, “… what did you roll?” makes no sense. Likewise, don’t go all Shakespearean Yoda and say, “Prithee, in what did you roll” No one talks like that, and if they did, we’ve kick sand in their face and take their lunch money. Just write like people actually talk. It’s grammatically fine.
Conversely, leave off the preposition in the sentence, “Where are you at?” Here, “at” is superfluous and shouldn’t be in the sentence at all. Saying “Where are you at, dude?” makes it no more correct. The word “at” is still unneeded. If you can take any word out of any sentence without changing its meaning, I recommend doing so.
2. Do Not Split Infinitives – Tell your Grammar nazis to boldly go to hell. There’s nothing grammatically wrong with it. However, and here is the key, you will get annoyed, well-meaning people who feel it is their job to emphatically correct you. It’s easier just to avoid it by writing, for example, “to go, boldly, where no blah has blah blah blahed.” Or, better yet, do a Stephen King, and go all Cujo on the likely infinitive splitter, the adverb, and bloody take it out. “To go where no one has gone before,” for example.
I could add the rule “take out all adverbs” a la Stephen King here, but I won’t because it’s not a fake grammar rule, it’s a fake style rule. Instead, I’ll say use adverbs only when emphasis is needed and when the adverb is more precise (and concise) than other words.
3. Don’t say, “I’m Doing Good.” This one is courtesy of Grammar Girl. I will refer you to her post for details, but suffice it to say am (to be) is a linking verb that doesn’t always require an adverb. You can use adjectives after linking verbs. When the nazis correct you, correct them back. They never even heard of linking verbs.