May I have some adjectives that collocate with the word "problem" meaning difficult or so? I've already looked it up in Oxford Collocations but didn't find adjectives having the same meaning as difficult.
Thanks a lot.
Thanks. If you don't mind my question, may I possibly know why 'arduous' doesn't sound alright to you? I'd like to know what image it makes in a native's mind.
May I sneak in with the suggestion that although "arduous problem" does occur, "arduous" more usually has the sense of being strenuous and demanding physically (often mentally too), and to some extent prolonged. A journey, lifestyle or lengthy task are typically arduous.
To your list of problems I'll submit: tricky, vexing.
not a teacher
Thank you both for the very useful suggestions.
Please note I'm not a teacher nor a native speaker;
I'd add :
I'm sorry if I repeated some;
Some more adjectives:
(macmillan advanced learners dictionary)