The ones that surprised me most when I arrived here. English: I wish I had...American: I wish I would have..or I wish I would have went. Not to say that English people do not have their own problems.
I always wondered if the "I wish I would have" might have come from the large number of German immigrants to the US. Certainly, to this Brit whose profession is the English and French languages, quite a few constructions inAmerican English strike me as possibly due to non-English-speaking immgrants "ironing out" the more difficult and specific bits of British English. I haven't studied it however.
The subjunctive mood particularly messes with people. This allows the proper conjugation of "If I were to have..." "would" speech often falls here or other hypothetical tenses that get messy.
Why do regardless and irregardless mean the same thing, but reversible and irreversible don't? (yeah, I know, but it's a real word if people use it often enough)
Why do flammable and inflammable mean (approximately) the same thing, but combustible and incombustible are opposites?
I don't know if this is a carryover from Norwegian usage, but many people in the northern Great Plains don't use "loan" as a verb. They get a loan to buy a house or car, but they borrow a book from someone, or borrow a book to someone. They do not loan a book to someone. This quirk drives outsiders nuts.
Very true. I sometimes suggest that foreign speakers of English should avoid "get" altogether since it is so very idiomatic and hence susceptible to awkward errors.