You can ask questions about the theory, quizzes, and exercises in the discussion section linked in the right menu of each lesson.
You'll see a new topic creation form in the Discussion section, and below that, you'll see all your previous questions about the lesson.
Look for the answer to your question in other students' topics. It may be already there, so you won't have to ask and wait - you'll be able to figure it out faster.
If you still have questions, create a new topic. Explain what you don't understand, what you've tried, sketch your reasoning, and, if your question is about code, be sure to provide a code review. A more detailed question will help you get a good answer right away.
We'll answer within 2-3 days. Support staff and other students, as well as some of the more experienced people who can help, can see the topics in the Discussion section, and your question and answer will help everyone grasp the lesson's subject better.
Another good way to get an answer for your question is to ask in Hexlet's Slack chat.