1) You should never buy an Oxford dictionary because they are "too easy" (despite, you know, publishing the OED, THE English dictionary) (adv class-Danish)
2) The phrase "in conclusion" doesn't exist in English (adv class- French)
3) After a preposition, you must always use a gerund (the ing verbal-noun form), whatever the sentence (int class- Turkish - this guy wrote some magnificently confusing sentences until we got this straightened out)
4) You may not use any phrasal verbs in formal written English, so there's no point studying them at university level (phrasal verbs are verbs that come with prepositions, like put up with, some can be replaced with latinate vocab for formal writing, but it's not a good idea to overdo it)
Any other language teachers here have similar "facts" about their language your poor students got saddled with?