I played around with your product and I've found that I need to enter words or select missing ones like in Duolingo. It seems a bit more difficult than Duolingo though.
The biggest problem to me with Duolingo that it doesn't explain certain language rules (I am learning German) and you have to brute-force until you "get a feeling" and memorize patterns.
Another problem, is that there is very little focus on developing your speaking skills which (to me at least) are the most important.
I would really love to see some emphasis on speaking skill development.
Even more, Duolingo sucks at text understanding development - you get to learn sentence by sentence, but you are never getting a "full picture". So when you get a proper, full text (some short article, for example) in another language, it just feels a bit overwhelming.
Basically, what I am trying to convey and what's missing (for me) in this kind of apps, is that learning language is not about filling in missing words in the sentences.
Btw, I really like that you charge $8 - IMO Duolingo here misses out a huge chunk of money. I would easily pay $5 and probably up to $10 but now I have just to "endure" one ad every 5min which doesn't bother me at all.