Why is the prefix con- sometimes short, sometimes long?

Question

A friend sent me this image:

Her question was simple: Is the Latin any good? The Latin indeed is good, and if one accepts the English to be in LOLcat, the English checks out as well.

However …

I also added a comment, that it would be even better were the long vowels marked (‘though many consider that to not be a necessity’), which is when I discovered that the expected vēnī, vīdī, *cōnvēnī, cōnsēdī did not match what the dictionary said: vēnī, vīdī, convēnī, cōnsēdī; in fact, I assumed there was (yet another) error in the Wiktionary entry before I checked my dictionary, and indeed there are numerous words starting with the prefix cō̆n-, some of which should be long, some short. Why is this? What causes cōnsīdō to to have a long vowel whereas conveniō has a short one?

My immediate guess is that the vowel is lengthened due to the following present tense long vowel and that this is retained throughout the paradigm, but this is mere guesswork. In other words, I believe that this is not something to do with obscure Old Latin or Proto-Italic, but rather that this is an example of phonetic rules (that I do not fully understand) being at work. I would really appreciate an answer that can explain why this quantity change is at work and which rules (if any) govern this change.

Answer 1

We can consider the underlying length of the vowel in con- to be short: this is the default.

In cōnsēdī, and in the majority of cases where the vowel in the prefix cō̆(n)- is long, it is a result of the regular lengthening of any vowel before ns or nf. The same lengthening causes in-s..., in-f... to be realized as īns-, īnf-.

Aside from that, I only know of one other, much less common context where the prefix cō- shows up with a long vowel: some words where the unprefixed form starts with the letter n, such as cōnectō, cōnītor. Some words with n instead take the form cog-, as in cognōscō, cognātus. Forms with con-n- are apparently a more recent alternative to cō-n.