Translation:She was putting the plates on the table.
I said "placing" the plates on the table and it was rejected. According to a multitude of online sources, this translation should be accepted. It's frustrating how unreasonable this specific course is.
Did you report it? http://context.reverso.net/translation/french-english/mettait
Did you say "She was placing the plates on the table?"
What was your complete sentence? "She had the plates on the table." would be wrong. "She had put the plates on the table." would be the pluperfect tense in French and not the imperfect. "Elle eût mis les assiettes sur la table." http://conjugator.reverso.net/conjugation-french-verb-mettait.html
It rejected "she was putting the dishes on the table". Reported 19 April 2018.
Assiette = plate, not dish. Dish = plat. Confusing because plat is a false cognate.
“Dish” has more than one meaning though. If it is a dish used to serve food to everyone or if it is referring to the course being served, then it is “plat”, but when you are talking about putting the dishes on the table, you could mean dishes for eating or plates, which would be “assiettes.” https://dictionary.reverso.net/french-english/Plat https://dictionary.reverso.net/english-french/Dish
You could try reporting it or "She was setting the plates on the table." might be better suited to imperfect. http://context.reverso.net/translation/french-english/mettait
Off topic. Duo never requests me to translate a sentence into French using the keyboard. I am always selecting word buttons. Is that a setting that I can change? Or I have to get to a certain level?