Substitute a word requires certain language knowledge. This should be tested somehow, so that the application knows the user level. For example, having no prior Spanish knowledge I would not do substitute a word, and i can do listen exercises which show me some common words and already teach me something. This was enjoyable... for the first 3 minutes. After just one run and 3 minutes into the registration it already…
The core features are free, then there are some advanced features you can pay for. I'm trying make as much free as possible, at the same time I'm bootstrapping everything and need some way to support the site. The cloze-listening is a trial that resets daily.