方便 has three meanings that can trip learners: (1) 'convenient' (adjective), (2) 'to help out / accommodate', and (3) the euphemism 'to use the toilet'. Context disambiguates:
你什么时候方便？ = when are you free/convenient? but
我去方便一下 = I'm going to the loo. Don't say
我不方便 to mean 'I can't make it' right after someone asks about your schedule or you'll get puzzled looks.
