Set groups of words that will be equal during the search.
Set synonyms groups easily right from the plugin settings page.
Yes. If you enable AND search logic and in your search query presents a word from one of the synonyms group you will still see search results that contain any of the synonyms.
No, there are no limits. But it is recommended not to use too many of them because this can greatly increase the plugin index table.
Yes. All plural forms of words work great with synonyms.
Download free version from wordpress.org repository.
Read about differences between free and pro versions here.