In dialogue, common usage (as distinct from actual bad grammar) is fine. But in narrative, it doesn’t matter whether “everyone says that”, if it’s wrong, authors shouldn’t be using it. The author needs to obey the rules of grammar.
“To boldly go” may be a catchy line in a movie, but split infinitives are incorrect.
Never say “different than”. It’s “different from”.
If you have two daughters you have an older daughter and a younger daughter. You don’t have an oldest (or eldest) daughter until there are three or more of them.
Fewer is if you can count them. Otherwise use less. Fewer chocolates in the box, but less coffee in the pot.
There is no such thing as a half a sudden, so you can’t have “all of a sudden”. The word is “suddenly”.
Outside of/inside of “She licked the inside of her lips” is correct. But she didn’t go “inside of” the house. She simply went “inside the house”.
Subject and verb must agree. A group noun takes a singular verb. “The flock of sheep is grazing”, “the crowd was waiting”.
These are rules. Don’t yell at me if you don’t like them. Look them up yourself in the Chicago Manual of Style: http://www.chicagomanualofstyle.org/home.html then obey them.