Translation:After eating, you must brush your teeth at once.
I was wondering about this when I first saw this comment as well. However, some sources appear to have shown that it is in fact better to wait before brushing your teeth.
If you've eaten an acidic food or drink, avoid brushing your teeth right away. These acids weaken tooth enamel, and brushing too soon can remove enamel.
I also found sources that state that it's better to brush before eating!
By brushing right after you eat, you will get rid of bacteria before they attack your tooth enamel.
Consequently, it's a good idea to brush your teeth before eating an acidic food and to drink a glass of water when you are finished to wash away the acids.
The sad thing is they've been aware of how stilted and bad the English is on this course for a long time and done nothing to remedy it. It's not like the criticism isn't constructive...people need to put their egos aside and accept that if a native speaker of a language says your translations are bad you need to change them for the sake of improving the teaching materials.
Amen to most of the comments below (non-dental)! And we native speakers of English would say "brush YOUR teeth" not "brush teeth." I'm sure we're all helping Duolingo enormously by reporting these idiomatic problems. Having to memorize the slightly wrong answer in English instead of focusing on learning Chinese is DEFINITELY still a big problem.
The translated sentence, though literal, is really bad and sounds more like Chinglish than English. They must be following the word order of the Chinese sentence very closely in the English translation.
I wrote "After eating, you definitely need to brush your teeth immediately" and it was rejected. Reported on February 9, 2019.
EDIT April 14, 2019: The new default translation is way better.
since there is only an implied subject, I must or you must or one must or we must. shouldn't all be accepted?
after eating meals, we must immediately brush our teeth. there is no indicator for "your" in the given sentence, though I understood the Chinese with ease, I now just failed out of a key lesson.