Dictionaries. 

Cambridge dictionary: https://dictionary.cambridge.org/

Linguee: https://www.linguee.es/

Wordreference: https://www.wordreference.com/

​

Synonym and antonym dictionary.

Thesaurus: https://www.thesaurus.com/ 

​

Idioms dictionary. 

The free dictionary: http://idioms.thefreedictionary.com/

​

​