title_body: How do Romance-based naturalistic conlangs deal with the different principal parts of a verb? The Latin verb has four principal parts (e.g., ducere, duco, duxi, ductum) that are sufficient and necessary to create all inflected forms and derived words from it. As far as I know, no naturalistic conlang attempts to preserve the French passé simple and related tempora in other Romance languages; so we can drop this one. Usually, the differences between infinitives and present tense forms are also levelled out by dropping most of the Romance verbal inflections.
  
  This leaves us with two principal parts, one for the infinitive, present tense and past tense, and one for the past participle (or at least for derivations from the past participle, in case that the participle is somehow regularised).
  
  I remember that Edgar de Wahl even went further in his design of Occidental-Interlingue using a device called de Wahl's rule (the description in the Wikipedia differs somewhat from my presentation) in deriving also the infinitive and the present tense stem from the past particple, so his word for "to lead" is ducter. He isn't 100% consequent, but this adds a lot of regularity to his design.
  
  How do other Romance-based conlangs deal with the principal parts?
upvoted_answer: I answer first for two of my own invented languages, Kerno and Loucarian. Since the question is now broadened to invented IALs, I choose to add several additional sections: Interlingua, Sabir, Occidental, Romanal, Medial Europan and Lingua Franca Nova. Also, just so one can get a flavour of these languages, I append the Pater Noster in each.
  
  Looking back at some of my old Romance languages, I don't think I paid much attention to the Latin principle parts as a system. As the languages evolve out of Latin and into their more modern selves, the systematic nature might fall apart or accrete new forms.
  
  In Kerno, we can certainly see the survival of at least three principal parts for the regular verbs.
  
  cantam, cantar, cantú (sing)
  caru, carer, carú (love)
  dormu, dormir, dormú (sleep)
  
  It's not until we meet up with those good old irregular verbs that more principal parts seem to linger:
  
  In the Grammar, there is actually a section that lists the principal parts of the irregular verbs (though for some unknown reason, I didn't call them such):
  
  doc (or doy), dar, dedai, doú
  istam, ystar, stetai, ystú
  currem, currer, coscorrai (or cocorrai), cursú
  llodu, lloder, llosès, llosú
  ioc, ir, fu, eú
  sentu, sentir, senti, sentú
  
  So, four principal parts surviving.
  
  a Phazeoir Nusteor que bias 'n y ceues;
  foreth noef il tew nom;
  gouenyes il tew camouils;
  foreth fès la teva gouoluntáts
  en lâ derra cuomo 'ny ceues;
  danos-el osdia le nusteor panèn cuotidièn;
  dimeti y nusteor dheuz
  cuomo dimitemus ai nusteor dheutoeres;
  et ne nus attrayer rhen al tentación,
  mays eliveránus des val.
  
  
  
  Loucarian takes its inspiration from Sabir and has a similarly reduced verbal conjugation. 
  
  amare, amando, amato
  clevere, cleviendo, cleveto
  ajire, ajiendo, ajito
  pejeire, pejiendo, pejito
  vidure, videndo, viduto
  
  While not a principal part of the ancestral Reman language, modern Loucarian takes the present/infinitive, progressive/continual &amp; past for its principal parts.
  
  nemet sospeito pôdis IC cata maccareire; etti pejeto IC:
  enamverver tim maccareire coudeiere ican:
  jenì abba en samayam na
  qedd al iccà nomon
  sôppere al iccà noummen podeqc festinam
  outtato wad al iccà alfas eim en al tir eim en al paradeisiam
  cvercare tim adis nasser hotim al jenìm pountim cascoumênem
  demetere tim al jenìn nobes eim nas demetere al jenìn namaderam
  mire doucere nasser adis peirasmom
  mire dê côire tim nasser ad injoudissiam.
  
  
  
  Interlingua is an invented Romance IAL. From the I-a grammar article under Verbs, we can see exactly what has been done with Latin's principal parts.
  
  
  All personal inflexion is stripped off, so there can be no "first principal part" as such. However, the Latin present stem remains largely intact, so the I-a present tense could continue to serve as first principal part.
  The infinitive remains distinct.
  The old Latin perfect stems have all been obliterated; the I-a past tense is formed upon the Latin imperfect (present stem), so there can be no "third principal part".
  The Latin past participle remains in I-a.
  
  
  One could say the principal parts of an Interlingua verb are:
  
  parla, parlar, parlate
  dice, dicer, dicíte
  senti, sentir, séntite
  
  Interlingua is not entirely regular as regards its verbal system. There are a few (unofficial) irregulars (thank Goodness!)
  
  The principal parts of esser are:
  
  es, esser, era, essite thus four principal parts.
  
  Nostre Patre, qui es in le celos,
  que tu nomine sia sanctificate;
  que tu regno veni;
  que tu voluntate sia facite
  super le terra como etiam in le celo.
  Da nos hodie nostre pan quotidian,
  e pardona a nos nostre debitas
  como nos pardona a nostre debitores,
  e non duce nos in tentation,
  sed libera nos del mal.
  
  
  
  Sabir (also knows as Mediterranean Lingua Franca) was an actual, natural Romance IAL in use within the Mediterranean Sea basin from the middle ages onwards to about the mid 19th century or so.
  
  It is well known for severe reduction &amp; levelling of morphology in general. There are but two principal parts, both surviving from Latin:
  
  parlar, parlato
  fazer, fazeto
  sabir, sabito
  
  The present tense, imperative and infinitive are all wrapped within the immutable infinitive; the past and past participle are wrapped up within the old perfect participle. 
  
  There are a couple odd forms. For example, in the Pater Noster we find noi volir ki nomi di ti star saluti of which I (having no grammar of the language available) can only imagine to be a past participle of saluer. Why the -i termination and why not "salueto", I don't know. Obviously Sabir was not designed to level all irregularity. It evolved naturally and over a long period of time within a broad geographical region. Oddities are bound to survive / arise.
  
  Padri di noi, ki star in syelo,
  noi volir ki nomi di ti star saluti.
  Noi volir ki il paisi di ti star kon noi,
  i ki ti lasar ki tuto il populo fazer volo di ti
  na tera, syemi syemi ki nel syelo.
  Dar noi sempri pani di noi di kada jorno,
  i skuzar per noi li kulpa di noi,
  syemi syemi ki noi skuzar kwesto populo ki fazer kulpa a noi.
  Non lasar noi tenir katibo pensyeri,
  ma tradir per noi di malu.
  
  
  
  Occidental is another invented Romance IAL. I really don't know much about it beyond a couple articles I've read online. Even so, we can easily pick out the remaining principal parts from a good sample text:
  
  da, dar, dat
  inducte, inducter, inductet
  veni, venir, venit
  
  As with Interlingua and Sabir, the morphology of person is eliminated, leaving only a present stem. Also, the perfect stem itself is eliminated. This leaves us three principal parts.
  
  Patre nor, qui es in li cieles,
  mey tui nómine esser sanctificat,
  mey tui regnia venir,
  mey tui vole esser fat,
  qualmen in li cieles talmen anc sur li terre.
  Da nos hodie nor pan omnidial,
  e pardona nor débites,
  qualmen anc noi pardona nor debitores.
  E ne inducte nos in tentation,
  ma libera nos de lu mal.
  
  
  
  Romanal was another invented Romance IAL. I have again no reference grammar, but a perusal of a text shows some interesting morphology.
  
  das, dar, dat
  
  Forms like ne nos inducas and dimitta speak of the survival of Latin negative imperative and possibly imperative forms as well. Romanal is well worth the visit for the spiffy verbal conjugation alone!
  
  Patro nostri,
  qui est en cieles,
  sanctificat estas nomine tui,
  advenias regne tui,
  fias volite tui,
  sicut en ciele, et en terre.
  Il pane nostri quotidiani das ad nos hodie,
  et dimittas nostri debites,
  sicut et nus dimitta debitantos nostri,
  et ne nos inducas in tentatione,
  sed liberas nos ex male.
  
  
  
  Medial Europan was another invented Romance IAL.
  
  It would seem, from the given text, that all distinctions of conjugation have been reduced to but one. We can see that the present tense and infinitive are distinct.
  
  pintit, pintir
  
  Un englo, un franco ed un deuto havit le taske pintir kamele. Le englo voyajit ad Afrike for studiir le kamele in tisui doimie. Le franco gidit al zoologi jarden, ed le deuto pintit on kamelo ex le profunde de sui psyke.
  
  
  
  Lingua Franca Nova was also inspired by Sabir. Its verbal system is the most reduced respecting the old Latin principal parts. That is to say, there are no principal parts, the bare verb stem being used for all tenses, in conjunction with temporal particles.
  
  leva
  come
  boli
  
  Nosa Padre ci es en la sielo,
  Ta ce tua nom es santida.
  Ta ce tua rena veni.
  Ta ce tua vole aveni
  sur la tera como en la sielo.
  Dona oji nosa pan dial a nos,
  e pardona nosa detas,
  como nos pardona nosa detores,
  e no condui nos a tenta,
  ma libri nos de malia.
  
  
  
  NB: I deliberately left out Esperanto / Ido because, while their vocabulary is largely paneuropean, its grammar is not Romance. (I've read several sources that claim it is closer to Slavic grammatically.) Also, I leave out Novial for the same reason, only its grammar is much closer to that of English.