The main difference between the Duolingo Basic (free) version and the Plus (with a subscription fee) version is that the latter offers users an ad-free experience and allows offline access. Also, the Plus version gives you an unlimited amount of hearts (equivalent to a gaming "life") as well as a more advanced tracking system of your progress, allowing you to determine and fine-tune the areas you need improvement on.  The Plus version likewise gives access to more extensive quizzes that test and give you a more comprehensive view of the level you are at, again as a progress meter of your learning.