Words are composed of roots and stems.
The minimal unit of meaning of a word, or the word's morpheme, is named the root. Since the morpheme, or root, is the simplest form of meaning of a word, it cannot be broken down further into more minimal parts. For example, the words "run", or "jump" are morphemes because you cannot examine them further within. They are entire and basic meanings of unit. To better list morphemes, just think about words that have nothing "attached" to them and are words that, if we were to add affixes to them, would change their role within a sentence, but the morpheme itself will not lose its original meaning.
The stem of a word consists of the combination of the root (discussed above) and the application of what is known as a derivational affix. A derivational affix is a bound morpheme, which changes a word from one category to another. For example, the noun "argu (e)" can switch to "argument" with the affix of the stem "ment". Hence, a stem is not a basic meaning of its own, but a combination the root as well as the form which will inflict onto the word a separate syntactic meaning. In the case of the root "friend", then think about all the affixes (stems) that could be added to change the syntactic meaning of the word: friendless, friendly, friendship, friendliness, etc. The combination after the transformation of the word becomes the stem.
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