What is the difference between english.stackexchange.com and ell.stackexchange.com?
Why two sites are built as English Language forums?
This site is for people who are still learning the language. More basics concepts are covered here to help learners understand and use the language comfortably with a native English speaker. The other one is structured more for native and fluent speakers of English where more advanced concepts can be covered.
Think of it as a difference between a forum for arithmetic and a forum for calculus.
english.stackexchange.com is for people who are fluent in or native speakers of English to discuss problems that all speakers encounter. ell.stackexchange.com is for learners of English to ask questions that fluent speakers would not need to ask. For example, your post on this site that I answered is not something that a native speaker would ask, so it belongs here.