These are amazing free online dictionaries: 1) https://www.ldoceonline.com/dictionary/ - LONGMAN Dictionary. English - Korean mode is available. Great for its integrated thesaurus, collocations, grammar tips. It also says what common mistakes you should aviod. I also like the way it's organized, it's really clear and user-frienldy. AND, it shows you whether a word is on the list of the top 1000, 2000, 3000 spoken or written words . 2) https://dictionary.cambridge.org/ - CAMBRIDGE dictionary. English - Korean mode is available Loads of most useful examples from the Cambridge English Corpus and the web are included in there. 3) https://en.oxforddictionaries.com/ - OXFORD dictionary, of course As brilliant as the Cambridge dictionary , lots of good examples, with lots of natural sentences. 4) https://www.macmillandictionary.com/ - MACMILLAN dictionary Excellent for studying synonyms. The words and phrases are grouped according to their meaning. Very convenient! The definitions are rather short though. 5) some more like Merriam-Webster. These days my favourite is Longman dictionary :) But I also have a paper Oxford dictionary (a rather old edition). As a language learner and a person who's always loved learning languages and somehow can speak three of them, I would highly recommend using a MONOlingual dictionary. Hope this helps!

For beginners, a bilingual dictionary is needed. For intermediates, a monolingual learners dictionary is useful. I use the Longman Advanced Study Dictionary in my class for immigrants. This dictionary has simplified explanations and the 3000 most common words are marked in red. For advanced students, typically university students in an English-speaking country, I recommend the Longman Dictionary of Contemporary English. This dictionary is a more complete learners dictionary.