https://owl.english.purdue.edu/

https://www.grammarly.com/l

https://www.powa.org/

http://www.economist.com/styleguide/introduction

http://grammar.ccc.commnet.edu/grammar/index.htm

https://www.nounplus.net/

http://www.paperrater.com/

https://leo.stcloudstate.edu/index.html

https://www.gramlee.com/

https://virtualwritingtutor.com/

http://proofreadbot.com/

http://www.grammarbase.com/

http://www.polishmywriting.com/

http://www.whitesmoke.com/

http://arts.uottawa.ca/writingcentre/en/hypergrammar

http://www.bartleby.com/141/

https://brians.wsu.edu/common-errors/