Are these sentences correct? :
I will take an exam on Wednesday. The exam can be difficult – I heard there were going to be some really hard phrases.
Or should I write: the exam could (or might ?) be difficult….
Are there some other mistakes?
'can' is fine. I'm not sure you can talk about 'hard phrases' -- complicated, unusual, formal, convoluted, obscure, puzzling etc. sound better to me. Did you mean 'sentence' by the way?
Are you all sure? Could someone other answer it?