Salvete omnes,
In the fourth principal part why do some books (and people) tell you to use the -us ending while others tell you to use the -um ending?
e.g. claudo, claudere, clausi, clausus/clausum
thanks
I really do not know. It does help to distinguish deponents and non deponents (deponents as a rule use “____us sum”). It seems to make more sense to use the -us ending since it is the first ending listed for an adjective. Perhaps the neuter is used so as not to discriminate between masculine and feminine by choosing the neutral gender, though that seems silly when talking about declension endings.
I would use the -us ending as -um is restricted to neuter endings of course and supine of motion 
I believe that the -um ending is traditionally listed because what is being listed is the supine, not the past participle (although they are identical in form). Of course, you will see the -us ending listed in verbs like audeo, audere, ausus sum, (where the participle is being listed in use with ‘esse’ in a perfect passive construction)
But to the original question - are you aware that the fourth principal part is a regular 1st/2nd declension adjective, and has all the different endings that these adjectives have? I only ask because you say that they ‘tell you to use’ one ending versus the other. The ending that you should use is based on the context - it always agrees with some noun in a given sentence.
Old Mr Bennett in his ‘New Latin Composition’ (downloadable from this site) describes the system he uses as follows:
”In giving the principal parts of verbs, the perfect passive participle is used instead of the supine; if the verb is transitive, the participle is given in the maculine form, otherwise in the neuter. When the perfect passive participle is not in use, the future active participle is given, if it occurs”.
You’ll find this in the footnote on p. 17 of the pdf file (which is a bit jumbled).
Incidentally, does anyone know how to convert this kind of scanned-in ‘old masterpiece’ (‘aere perennius’) text to editable text?
Cheers,
Int
‘maculine’? No, masculine.
Unfortunately, it really cannot be done. Technically, these pages are saved as images, not text. The only way to convert it to text would be either by hand (not appealing at first, but just think how much Latin you would know by the end) or to use character recognition software. My person experience with character recognition software is that it is barely a step above typing it yourself. Usually there is a very large percentage of unrecognized characters (and that’s with modern texts, I’m sure it would do even worse with the older font type in these texts).
That was really ‘acerbum nuntium’! 
Int