“You are mostly welcome.”
I first came across “You are mostly welcome” on a photo of a sign in one of those online collections of mistranslations, but since then I’ve had a client say it to me in real life.
It’s a completely logical thing for a non-native speaker of English to say: “most” can be an intensifier, and they probably know that adverbs should end in -ly, and that omitting -ly can be a marker of low-prestige or “bad” English, as in “real good” or “do it quick”. It’s not their fault that “mostly” illogically switches from being an intensifier to meaning “not wholly”.
“I never stop to think about interesting mathematical problems.”
This was from a foreign student’s university application that I was editing. I corrected it to “I never stop thinking about interesting mathematical problems.”
Non-native speakers often struggle with the difference between “to think” and “thinking”, but if they get it wrong it usually just sounds a bit unnatural, rather than completely reversing the meaning of the sentence, from conveying “I think about mathematical problems all the time” to conveying “I never pause the other things I’m doing in order to think about mathematical problems”.
Even more illogically, “cease” is a synonym for “stop”, and “I never cease to think about interesting mathematical problems” is completely fine!
The difference between “have to” and “get to”.
“I have to spend the whole afternoon with Katie” implies you see spending an afternoon with her as a chore, whereas “I get to spend the whole afternoon with Katie” implies you see spending an afternoon with her as a treat.
The same distinction applies in the past tense: “I had to” implies you didn’t want to; “I got to” implies you did want to. So far so good.
What complicates matters is that “have got” is generally synonymous with “have” – “I’ve got a cold” means the same as “I have a cold” – so “I’ve got to spend the whole afternoon with Katie” means the same as “I have to spend the whole afternoon with Katie”.
So you have these two almost-identical sentences:
“I got to spend the whole afternoon with Katie.”
“I’ve got to spend the whole afternoon with Katie.”
One of them means “I spent the afternoon with her in the past and I’m happy about this” and the other means “I’m going to spend the afternoon with her in the future and I’m not happy about this.” They have opposite emotional valences and different tenses, even though they only differ in writing by two letters and an apostrophe (‘ve), and they only differ in speech by one phoneme ([v]). But the most confusing part for learners of English is that the difference between them (‘ve or have) is normally a past tense marker (consider “my feet hurt” versus “my feet have hurt”) – but in this case, the sentence without it is in the past tense and the sentence with it is not!
(And in some kinds of informal or sloppy speech, “I’ve got to” can be reduced to “I got to”. So “I got to” might be completely ambiguous between “I did in the past and I liked it” and “I’m going to in the future and I don’t like it”. You can distinguish them sometimes by the way that the latter can be reduced to “I gotta” and the former can’t.)