Are there any sort of patterns for the 3rd and 4th principal parts of the 2nd, 3rd, 3rd io conjugations?
laudo, laudare, laudavi, laudatum - 1st conjuation verbs all follow this model. Is there anything to keep in mind for 2nd, 3rd, 3rd io?
Or do you just have to memorize each principal part set for each and every verb?
Thanks,
Robbie Moran
Yes, there are actually, and it is odd that most textbooks do not discuss them. While, all but the 1st conjugation are regularly irregular, there are some recurring themes. This tutorial I wrote goes over the irregularities of the principal parts pretty thoroughly: http://www.textkit.com/tutorials/20030923-perfect-stems-page1.php?aid=3&tid=3
Basically…
1st conjugation is highly regular: -o | -are | -avi | -atum | (laudo, laudare, laudavi, laudatum)
2nd conjugation: -eo | -ere | -ui | -itum | (moneo, monere, monui, monitum)
3rd conjugation: -o or -io | -ere | anything that ends in -i, but usually -di or -si or -xi | anything that ends in -um, but usually -tum or
-sum | (contendo, contendere, contendi, contentum; dico, dicere, dixi (dic-si), dictum)
4th conjugation is highly regular: -io | -ire | -ivi | -itum | (munio, munire, munivi, munitum)
Unfortunately for us, there are many exceptions to every conjugation, and you do have to memorize every principal part of every verb… there is no way around it 
Be grateful you are not learning Greek (unless you are) because you would have to memorize, I think, 6 or 8 parts instead of 4.
Hi Robbie,
As you’ve said, the first conjugation is the most regular. The most common pattern for 2nd conjugation goes:
moneO, -Ere, -uI, -itum.
The 3rd conjugation is something af a free-for-all, however there is a tutorial on this very site that will help you identify some of the more common patterns of 3rd pps. Click on ‘Tutorials’ on the main page, and find the one called
‘Formation of the Perfect Stems: Why are they so unusual?’.
The 4th conjugation is fairly regular:
audiO, -Ire,- IvI, -Itum
Also, sorry to relate, not all 1st conjugation verbs follow the rule you mention. There are some (can’t think of one at the moment, but I know they exist) that go (stem)-o, -are, -ui, -itum.
Cheers
Phil
Some odd perfects and supines of verbs with infinitive in -are:
lavo, lavare, lavi, lautum OR lotum OR lavatum
seco, secare, secui, sectum
sono, sonare, sonui, sonitum
veto, vetare, vetui, vetitum
do, dare, dedi, datum(and compounds)
iuvo, iuvare, iuvi, iutum
sto, stare, steti, statum
tono, tonare, tonui, —
~dave
These two are taken from the first few chapters of Harrius Potter:
implico,-are,-ui,-itum OR -avi,-atum; to involve/implicate
obsto, -are, -iti; to obstruct
apologists of HP unite! 