> What gets me as a non-native speaker far more than "a vs an" is the "a vs the" conundrum. I still don't quite get when something needs to be "indefinite" and I've been speaking English for close to forty years.

"a vs the" is quite easy IMO.

Most non-native however do not use "that" and "which" properly though. "Give me the floppy disk that is blue" and "Give the floppy disk, which is blue" are two different things.

My wife is Japanese and fluent in English but still almost always mixes up the and a. I think part of the problem is that it just barely sneaks under the radar of a mistake that is bad enough to confuse English first language speakers in everyday use. So theres no impetus to correct it

Nobody says that. "Give me the blue floppy disk"