Stems/Roots/Combining Forms are fundamental.  They are indispensable in achieving success in the Bee and in obtaining an understanding of languages.  If you want to succeed, you should start learning these as soon as possible.  A list of all combining forms, suffixes, and prefixes in Merriam-Webster can be found here:

A Dictionary of Prefixes, Suffixes, and Combining Forms from Merriam-Webster’s Third New International Unabridged Dictionary

Keep in mind that this list does not contain all of the important roots to know, just the prefixes, suffixes, and combining forms.  For example, you won’t find the important Latin root bellum, meaning war, in that document.

You would, however, find that on the next list.  This is the list I used when preparing for the bee.  It contains 672 different stems, roots, and combining forms.  Mastery of this list would give you a very strong grip on language roots.  Without further ado, here are some of the most important Stems/Roots/Combining Forms to know:

Stems, Roots, Combining Forms

Some of the roots on this list may actually be too obscure for pragmatic use for a newer speller.  If this is the case with you, you should definitely make sure you have the following 100 most basic stems, roots, and combining forms mastered before moving on to anything more advanced.

100 Most Common Roots, Stems, and Combining Forms