A Website for Studying English as a Second Language
http://www.manythings.org/

YourDictionary is the easiest-to-use online dictionary. It is a dictionary you can understand with lots of tools to help you choose your words precisely, understand a word, avoid usage pitfalls and be confident in your use of words.
http://www.yourdictionary.com/

Common Errors in English Usage
https://brians.wsu.edu/common-errors/