Yet can introduce a contrast, refer to the future, or to something that you need more of. You can like a class, yet feel it's too hard. You may need to study hard yet again to try to pass a re-test. And even if an exam hasn't happened yet, you might worry it will be a disaster.
Yet is often used like but when connecting two ideas that contrast — as in, "I love apples, yet I hate applesauce." When yet is not being used as a contrast, it means "more" or "in the future" (which is really just more time). If you've been rehearsing for the school play for weeks, but still don't know your lines, you'll need yet more rehearsals if it's going to be a success.