I have given many links on my site, some authentic, some to use with caution. openburhan.com is a site for roots and words in the Quran, but use caution, for the owner of the openburhan.com looks like being under the influence of submitters/ rejectors of Hadith.
http://www.openburhan.com/
While
http://www.tanzil.info/ and
http://www.allahsquran.com/ also give Quran verses containing different words from a root, in this one individual words are also click-able. When clicked, this give a list of different words derived from that root.
A word of warning: This site needs to be evaluated as in many cases it says that the root of the word is not found, whereas even for me, the root is clear. for example, it lists
وَ صدُّوْا as without a root, while the root is
ص د د Secondly, although it gives access to a lot of translations, some of these are very deviant ones. Third, many words are listed as "null", but a good point is is that in such cases it points to the corresponding word in
http://www.corpus.quran.com which is an authentic site.
In the roots list are letters I for INITIALS (here used to describe
huroofe muqatteaat), and U for UNKNOWN roots). It says the unknown here are 78 in number.
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