It doesn't get any better when the individuals are supposedly native English speakers. Years ago I was teaching EFL and trying to get one of my more "relaxed" students to understand the different ways we treat singular from plural (e.g. "he is here, they are here"). After about a week of trying to impart this distinction, the student proudly came to me with a large poster for a local Oxford amateur dramatic society presentation of Rosenkranz & Guildenstern Is Dead!