Online Dictionaries:
* Merriam-Webster Thesaurus: [https://www.merriam-webster.com/thesaurus/certain](https://www.merriam-webster.com/thesaurus/certain)
* Oxford Thesaurus: [https://www.oxfordlearnersdictionaries.com/definition/english/certain](https://www.oxfordlearnersdictionaries.com/definition/english/certain)
* Dictionary.com Thesaurus: [https://www.dictionary.com/browse/certain](https://www.dictionary.com/browse/certain)
Online Thesaurus Websites:
* Thesaurus.com: [https://www.thesaurus.com/browse/certain](https://www.thesaurus.com/browse/certain)
* Power Thesaurus: [https://www.powerthesaurus.org/certain](https://www.powerthesaurus.org/certain)
Other Resources:
* Your own personal dictionary or thesaurus: If you have a physical dictionary or thesaurus, check the entry for "certain."
* Word processing software: Most word processing programs like Microsoft Word or Google Docs have a built-in thesaurus feature.
Remember:
* The best synonym for "certain" will depend on the specific context in which you are using it.
* Pay attention to the nuance and connotation of each synonym you choose.
* Use a variety of synonyms to avoid repetition and add interest to your writing.