Why does the length of a vowel before verb endings change?

Question

I'm learning Latin and I see that the stem I am supposed to add things onto keeps changing from long to short and back again. For example, take teneō, tenēre, tenuī, tentum.

As I see the present active first person singular is teneō, I might expect the e to always be short. However, I go to the second person singular and see tenēs. The third person singular is tenet. There is no pattern that I can see, as far as I'm aware. How do I know if the stem vowel of a verb will be short of long in a conjugation (not just present active)?

Answer 1

Most people just memorize it. There are only four patterns to learn—all first-conjugation verbs will go -ō -ās -at -āmus -ātis -ant, for example—so this tends to be enough.

From a historical-linguistics perspective, though, we know that the vowels in the first, second, and fourth declensions were originally all long. But sound changes in the history of Latin sometimes shortened them: before another vowel (habĕō, audĭō) and before certain coda consonants (amăt, amănt).

So if you want to know the general rule, the stem vowel in the first, second, and fourth conjugations is always long unless:

• It comes before another vowel
• It comes before a final r, m, or t
• It comes before nt anywhere

The stem vowel in the third conjugation, on the other hand, is always short, unless it comes before ns or nf (which lengthen preceding vowels).