Better reference site for English Writing-Synonym (同義字)

Whenever, you want to write something, you need more especially you are not native speaker of English.  My English writing instructor always wish me not to use same verb within one paragraph.  Well, that request push me to figure and learn what might be the appropriate synonyms. 

Here, I will present two sites for whom want to upgrade his/ her vocabulary database.

English Synonym Dictionary
1. https://www.thesaurus.com/
   This site is well structured and easy to read.

2. http://www.wordreference.com/synonyms/
   For wordreference.com, you can say this site designed in "google style". Simple but offer tones of information.