Verb tense suggestion
One of the great things about DuoLingo is that it teaches you most or all verb tenses for a language.
One way to really hammer them home would be to have a separate unit, after every unit that introduces a new verb tense, that mixes exercises in the new verb tense with all verb tenses learned previously.
For example, after the "imperfect" unit, there would be a general "verbs" unit with exercises in both perfect and imperfect. After the "perfect" unit, a "verbs" unit with exercises mixing perfect, imperfect, and present. Etc. Same with subjunctive, conditional, etc., as applicable depending on the language.
While the current units teach quite well how the different tenses (and moods) are formed, they train you to think of only one tense at a time. Mixed units are necessary to learn to be on the lookout for different tenses and to decide when to use which tense.