Prepositions:
As a non-native speaker, I sometimes struggle with prepositions – not things such as “under the table” and “on the bed.” Those are easy. But other prepositions can be tricky. There seem to be few hard-and-fast rules, and when there are rules, they have dozens of exceptions.
For example, it took me a while to figure out that apparently, you sit ON chairs with harder surfaces, IN more comfortable chairs and also in the driver’s seat of a car… but you sit ON bigger pieces of furniture like a couch or bed, no matter how comfortable they are.
Also, you travel IN a car or truck, but ON a bus, plane, ship, or train.
I also struggled with AT vs. IN for a while. Sometimes, people say they are “at the hospital” and sometimes “in the hospital” or “at school” vs. “in school,” etc. My impression is that “at” seems to refer to the general location, while “in” means the inside of the building.
I figured out that you apparently say ON Maui (an island), but IN the Bahamas (a group of islands).
And you say that something is ON top, but AT the top of the list.
Sometimes, an object is AT the edge (when it’s the edge of a surface or area), while others are ON the edge (when it’s a sharper edge).
Word creations
Sometimes, I’ve been known to create new words – because apparently, the English language doesn’t have enough 
I puzzled my beta reader with creations like “resignated,” “honorful,” “close-minded,” and “seldomly.”
Grammatically correct vs. naturally sounding:
I’m not sure whether it’s an advantage or a disadvantage, but since I live in Germany, I don’t hear the English language every day. I learned what I know mostly from books, including grammar books. In everyday usage, many native speakers use incorrect grammar without even being aware of it. You hear something so often that it begins to sound right. For example, 90% of all lesbian fiction novels seem to use “like” and “as” / “as if” as if (pardon the pun) they were interchangeable.
So, sometimes, I have to make a decision between using the correct form of a sentence or choosing the incorrect form because it sounds more natural to native speakers. In dialogue, the choice is easy. But what about narrative? Do you use proper grammar, or do you write from a deep point of view in which you use the grammar and diction of your POV character?
If any non-native speakers are reading this, I’d be interested in what kind of problem areas you have when it comes to writing in English.
