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)?