• Cambridge online
  http://dictionary.cambridge.org
  
  

• Oxford Dictionaries
  https://www.oxfordlearnersdictionaries.com
  
  

• Merriam-Webster
  http://www.merriam-webster.com
  
  

• Dictionary.com
  http://dictionary.reference.com
  
  

• Longman online
  http://www.ldoceonline.com/index.html
  
  

• Vocabulary.com
  https://www.vocabulary.com/
  
  

• Using English.com (Idioms & Idiomatic Expressions)
  http://www.usingenglish.com/reference/idioms
  
  

• The Free Dictionary (Idioms)
  http://idioms.thefreedictionary.com
  
  

• Idioms
  http://www.eslcafe.com/idioms
  
  

• Using English.com (Phrasal Verbs)
  http://www.usingenglish.com/reference/phrasal-verbs
  
  

A thesaurus contains lists of synonyms (words with the same meaning) and antonyms (words with the opposite meaning). 

• Thesaurus.com
  http://www.thesaurus.com
  
  

• Merriam-Webster
  This site can help you find out which phrasal verbs collocate with specific words or topics.
  https://www.merriam-webster.com/thesaurus

• Wikipedia
  https://en.wikipedia.org/wiki/Main_Page
  
  

• Reference.com
  http://www.reference.com
  
  

• Encyclopedia.com
  http://www.encyclopedia.com

• Grammar Overview
  This website provides an overview of various grammar items and types of sentence structures.
  https://academicguides.waldenu.edu/writingcenter/grammar/home
  
  

• Online English Grammar
  http://www.edufind.com/english-grammar/english-grammar-guide
  
  

• Purdue OWL - Grammar
  This site provides resources and exercises to help you use correct grammar in your writing.
  https://owl.purdue.edu/owl/general_writing/grammar/
  
  

• Writing Explained – The Writer’s Dictionary
  This site is a comprehensive grammar and writing resource that covers a wide range of grammar terms, concepts, and writing techniques.
  https://writingexplained.org/grammar-dictionary