There are forms of verbs in the 6th level excercises - e.g. imperatives and future - that are not discussed in previous lessons. Is this only my problem?
Although Duolingo provides some grammar explanations, not everything is explained in detail yet. You may want to look for some grammar reference elsewhere, read discussions or post your specific questions in your language section.
We're always working on the way that people learn grammar concepts through translation. If you do have language related questions, as Olimo mentioned, you can add a thread in the specific language discussion and get the community to weigh in.
I don't know what is meant by "6th level exercises", but I do get practice sentences which have verb tense forms that I have not gotten to in my tree. I get Imperatives and future sometimes. It doesn't happen often. This generally happens when I am practicing skills.