Student or Learner
When something was said and it was not clear to you... is it right to say "I couldn't understand you." ? Or should it be "I can't understand you."
Please explain why. Thanks
Drifter, I'm going to be bold and step on new2's advice. 'I didn't get you' is a slang expression and hence not regarded as 'proper English' (I use the term proper in its broadest sense).
In fact, in most dialectual vernaculars in the UK, 'I didn't get you' would lead to the puzzled reply: I didn't understand you! The latter is as it should be, and moreover 'I couldn't understand you' implies that there was some reason for not doing so, that either there was too much noise when the person was speaking, or that their accent was difficult to comprehend, etc.
That's fine and acceptable to most company, but still colloquial.
Yes, both are good and 'could you say that again, please' would be a more formal way of getting around the problem; another idiom we have here is 'come again?'
Essentially, couldn't is past tense and can't is present tense.
Past tense: I didn't take your advice (yesterday) because I couldn't understand you (yesterday).
Present tense: I am not listening to you any more (now) because I can't understand you (now).
Forget the slang for now - there are too many English speakers who learn slang first and then can't speak or write decent English when they need to.