For nouns, show plural form when teaching.
Often, if I want to know the plural form for many of the vocab words, I have to look it up separately in another dictionary. Even if I go into the vocabulary list and click on a specific word, sometimes Duolingo only shows the forms that were used in the lesson(s).
PS: I'm using the German course.
Since we use sentences to introduce new words to you, it would require us to give you more sentences in a lesson where we teach you some word. However, we should be able to make the vocabulary page show both the singular and plural forms of nouns, similar to how we show the conjugation of verbs.
As a German teacher I forced my students to learn any noun with article and plural, because there are so many exemptions to the rules, that I decided learning the rules was a waste of time. Using the same system for both verbs and nouns would be an immense step forward