A list of common different spelling-variants

Question

The beginner (like me) might occasionally find a word he is not familiar with. It is usually easy to retrieve the base-form (nom. or stem for a verb) and find it in a dictionary. Even more, there are tools today (like Wiktionary or William Whitaker's Words) that exact the base-form themselves.

Yet sometimes it simply does not work. I suspect in many cases this is because the author adopts a different spelling than the regular. Examples:

1. appellarunt for appellaverunt (this kind of shift - like others - is supported in Words).
2. Adding "st" (which I still not sure as to what this signifies) as in "agitandumst vigilias (Pl. Trin. 869)
3. Real-life example: The word praecipe in "Occumbunt multi letum ferroque lapique aut intra muros aut extra praecipe casu" (Enn. Ann. 391 Vahl).
   
   If you search praecipe using the tools mentioned, you get the imperative of praecipio which is clearly not the case here. It rather looks like the abl/adv of "praeceps" - I assume it is a some kind of spelling derivation(?), that might occur in other cases as well.

So the question is: Is there a kind of list of spelling contractions/suffix/prefix that are less-regular, yet common enough?

Edit:

I now see that Perseus supports the first two examples

Answer 1

Partial answer:

Your first example is called a contracted perfect or syncopated perfect.

When the perfect stem of a verb ends in a vowel plus V, and the ending starts with a short vowel, the V and the first vowel of the ending can be removed. For example, amāverunt > amārunt "they loved", delēvisse > delēsse "to have removed", audīveris > audīris "you might have heard".

Occasionally, the V disappears, but the second vowel doesn't. In this case, both vowels end up short (*). This is most common in poetry; Vergil for example often uses forms like audieram (from audīveram, "I might have heard") to fit the meter.

Perseus is generally very good at recognizing these, so I'd trust that over Whitaker's Words.

(*) In native Latin words, a vowel before another vowel is almost always short. When you see a long vowel before another vowel, it's usually in a loanword from Greek, like Aenēas. See this question for more information.

Your second example is elision. When one word ends in a vowel (or a vowel plus M), and the second word starts in a vowel (or a vowel plus H), one of those vowels disappears and they're pronounced as a single word.

This is most important in poetry, and usually isn't written out: the middle of Aeneid I.3 is written multum ille et but pronounced mult' ill' et. In almost all cases, the first vowel is the one deleted. (And you probably know all this already if you've done anything with Latin poetry.)

However, if the second word is est, the second vowel is deleted instead. And this is often written out explicitly: multum est would be written multum'st or multumst. If you see an unexpected st at the end of a word, it's a contraction of est. (If it helps, you can think of it in the same category as the enclitics -que, -ve, and -ne.)