Literature review

*Disadvantages:

• 
  How to learn speaking skills is not practical
  

Duolingo lessons all use pre-recorded or computer-generated voices, so sometimes learners will be limited in learning how to reflect and communicate in real-life contexts. Although the application has also included sample listening and repeating sentences to help improve speaking skills, this does not improve the problem because in some cases the technology does not have enough data and accuracy to recognize. whether the learners' pronunciation is really good and easy to understand.

• 
  Can't choose vocabulary topic
  

Because learners have to complete all lessons on the same topic in one Unit, then they can move on to another topic in the next Unit, so learners will not be able to choose and learn vocabulary for the topic that they really are. need.