Quizlet used to be a really great site. It had a lot of features that allowed students to study easier and allowed teachers to assign work easier. However, in an effort for people to buy into a monthly subscription, Quizlet has been intentionally locking features behind paywalls. First, they made you see more ads. Then, they made it so ads would interrupt your studying entirely. Now, different modes that were previously accessible to anyone using the site (such as Spell and Learn) are locked behind the monthly subscription.
Students now have to pay just to get good grades, and teachers can't use the site anymore.