I am currently using Duolingo for Czech and Dutch. Mind you, I speak both languages and I've been looking for a program that would help me refresh what I know. So when I found Duolingo, I was pretty excited. I'll go over some of my points to highlight the pros and cons of Duolingo.
- Interface. Personally, I think Duolingo has a fun user interface that allows for easy navigation for new learners. This also brings me to the layout. Each language has a "tree" which contains all the lessons Duolingo has to offer. You start simple with greetings, but this progressively turns into forming full sentences.
- Price. Duolingo is totally free of charge.
- Language diversity. Duolingo has many languages that range from Spanish to Klingon. What's great about the site is that you can add as many courses as you like for free.
- Feedback. Duolingo has a great feedback system in that it shows you what you missed on the translation. It has you correct the sentence at the end of the lesson. The great thing about this is that if you don't do this, you're unable to move on to the next lesson.
- Teaching. Personally, I'm iffy on this point. Duolingo does introduce an extensive amount of vocabulary to new speakers, but what it doesn't do is teach you the grammar you need to know to use that vocabulary.
I can conclusively say that I recommend Duolingo. It's fun to learn from and is user friendly. However, I wouldn't solely rely on Duolingo to learn a language. Some great sites I've found over the past year are FluentU articles, IWillTeachYouALanguage and some other ones I'll list below for reference.
https://itunes.apple.com/app/id*******
https://www.iwillteachyoualanguage.com/
https://www.babbel.com/
https://www.memrise.com/
https://www.101languages.net/
*The pictures below show the extensive amount of languages Duolingo has to offer. These are the results for courses specifically for English speakers.*