Why writing answers on a public forum like Quora is the modern way to teach english?
First, to make anything public will require the user to edit his/her materials many times. This will sharpen their arguments and contents, they will make more research before answering. More research equals more reading equals more knowledge gained. Meanwhile while researching and reading other materials (from Quora itself too), they can learn english, they reads more english sentences than their parents! There is this effect that when you publish a work/ answer to the public like giving a speech or writing an article for a newspaper, you will make your content better, when there are viewers, you will feel engaged and more responsible about your content.
Second, there are damn lots of good advices and insights in Quora answers.
Third, you can earn money through Quora answers. That is, if you have your own website. Self promotion on Quora can leads to extra traffic to your website, hence earning more money from Google Adsense from more viewers viewing ads. By self promotion, I do not encourage copy and pasting answers to generate views, but I encourage sometimes link back to your website or writing a short answer at Quora and make viewers view longer answer on your website.