In LLPSI Cap. XXII Orberg introduces supine endings and says in the accompanying student's manual 'Latine Disco' the following: The supine endings -um and -u are added to a modifie stem-form, the so called supine stem, which is also used to form the perfect participle - and the future participle. The supine stem is regularly formed by the addition of t to the present stem, e.g. saluta-: salutat-; audi-: audit-; dic-: dict-; in e-stems e is changed to i, e.g. terre: territ-; and there are several other irregularities, especially in 3rd conjugation verbs , here the addition of t may cause changes by assimilation, e.g. scrib-: script- (p is voiceless like t), claud-: claus- (dt > tt> ss> s).
When you know the three verbal stems, (1)the present stem, (2) the perfect stem and (3) the supine stem, you can derive all forms of hte verb from them. Consequently, to be able to conjgate (i.e. inflect) a Latin verb it is sufficient to know three forms, or 'principle parts', in which these stems are contained. Most useful are the three infinitives:
1. The present infinitive active, e.g. scribere
2. The perfect infinitive active, e.g. scripsisse
3. The perfect infinitive passive, e.g. scriptum esse
These are the forms of irregular verbs that will be given in the margin when ever needed (the 3rd form will be without esse, or missing if the verb has no passive, e.g. posse potuisse;....
Now this is a bit confusing. Is he saying that the form of irregular verbs given in most dictionaries and, indeed, by himself up until now is actually of no use and that 1, 2, 3. above is what you need?? Also true to his word he provides in the margin: vincire, vinxisse, vinctum for where any dictionary / 501 Latin verbs etc will give you vincio, vincire, vinxi, vinctum.
So my question is - er - what on earth is he on about? Is he saying that if I don't have vincire, vinxisse, victum but do have vincio, vincire, vinxi, victum that I will not be able to form the supine.... I don't understand.