On no condition should they swim at night is not a correct English sentence. It would make sense if the words i've underlined were In no circumstances.
On no condition should they swim at night is not a correct English sentence. It would make sense if the words i've underlined were In no circumstances.
With the change form 'on' to 'under', it works in AmE (as also noted in the dictionary entry'. I'd expect to see the plural form of 'conditions', though.