In our recent discussion, we examined the phrase 'I didn't know that you can't swim' and questioned its grammatical validity in English. It turns out, while it is commonly used, it's not entirely flawless. The correct version should be 'I didn't know that you couldn't swim', as the past tense ('could not') corresponds with the past tense in the first part of the sentence ('did not know'). Yet, the original phrase is still widely accepted due to its clear meaning and frequent use in everyday conversation. Remember, language evolves, and sometimes, what's technically incorrect becomes a part of our colloquial language.