Arsthbt

I am wondering if the stem of every word has an exact form?

For example:

• For the word genus, how could you determine is it gen or gener?


• For the word līber, how could you determine is it līber or lībe?


• For the word videō, how could you determine is it vide or vidē?

Unfortunately, there's no foolproof way to predict a noun stem from the nominative form.

Fortunately, you can predict the stem from the genitive form. So good Latin dictionaries will list both: your examples would be listed as genus, generis and līber, līberī.

The genitive endings are predictable, and also tell you which declension the noun belongs to: first will be -ae, second will be -ī, third will be -is, fourth will be -ūs, and fifth will be -eī.

Since you've added verbs, there's a trick to that too. Dictionaries will list both the citation form, videō, and the present active infinitive, vidēre. The latter is more informative on where the stem ends and the ending begins.

The concept of a "stem" is not completely clear/unambiguous.

Some Latin words have to be analyzed as being built on at least two stems: this is called "heteroclisy". One clear example of a heteroclite Latin noun is vās: the ablative singular vāse is built on the stem vās-, while the genitive plural vāsōrum is built on the stem vāsō-. (Historically, the explanation in this case is that the different stem is derived from a "collateral" form of the noun; Lewis and Short says that in anteclassical Latin, vāsum existed as a separate nominative singular form. But in Classical Latin, the stems vās- and vāsō- came to be used in complementary contexts, which allows vāse and vāsōrum to be analyzed as forms of a single noun. I've generally seen this noun described as having the stem vās- in the singular and vāsō- in the plural, although the nominative/accusative plural vāsa is formally ambiguous: it would look the same regardless of whether it represented vās- + -a or vāsō- + -a).

A number of other Latin nouns may or may not be analyzed as "heteroclite" depending on how you think of the idea of a "stem". This issue came up in a discussion I had with Joonas Ilmavirta beneath the answer that he posted to the question What consonants can a noun stem end in? Many third-declension nouns show an i in the genitive plural (before the ending -um) but not in most of the other forms (e.g. vermis has the genitive plural vermium, which seems to be built on the stem vermi-, but the accusative vermem and the ablative verme, which seem to be built on the stem verm-). The situation gets even more complicated if you try to include nominative singular forms in your analysis; for this reason, I've seen a number of pedagogical texts that don't attempt to relate the nominative singular form to a stem, but that just present the nominative singular as a form that must be memorized as a whole. Nonetheless, from an etymological perspective, almost all Latin nominative singular forms can in fact be analyzed as being composed of a stem and a suffix. I've also seen attempts to analyze Latin nominative singular forms as being synchronically composed of a stem + suffix (usually -s): for example, the nominative singular form rēx be seen as being built on the same stem rēg- as the other forms of this noun (rēg- + -s becomes rēx according to known rules for combining Latin sounds). However, I don't remember seeing an approach that tries to explain away the vowel alternations found in words like genus vs. generis in a similar fashion.