It works well to learn the basics, which means a few thousand words and the core grammar. Like ANY tool in language learning, it's not a complete solution.
The ONLY way to speak a language is to...gasp...practise speaking and use other resources! Ahhh, the horror!
Your father isn't trying. He's just using Duolingo as a game.
This is not entirely accurate. The better courses on Duolingo teach in excess of 7000 words (8000 for french) as can be seen on DuolingoData[1] and clicking on the W column to sort the courses by words taught.
Clozemaster is a good resource for vocabulary building if you want to supplement Duolingo.[2]
Run, running, ran,... Gosto, gosta, gostamos, gostou, gostei ....
Which I do not. Though your count might be more accurate.
fucking lol, the audacity
I speak already most of the main languages Duolingo teaches and the other courses are pretty weak