Ask questions that make your teen think
You don't have to be an expert on a subject to help your teen learn it. Just ask a few questions. For history, you might ask, "What put the events you are learning about into motion?" For math or science, you could ask, "Can you summarize an idea that was new to you from this chapter? How does it relate to what you already know?" If your teen can't answer, you'll both know she needs to review the material further.