Salvete, amici. So I’ve decided to read Ovid’s Fasti to educate myself a bit on Roman religion, and I’m struggling with an elegiac. It reads est tamen et ratio, Caesar, quae moverit illum / erroremque suum quo tueatur, habet. It’s referring to Romulus, and granting him pardon for being so warlike and so ignoring the stars, only dividing the calendar into ten months rather than twelve. I want it to mean something like this: "It is however also the reason he possesses (habet loosely referring to ratio?), Caesar, which moves him, and for which man (is quo just stressing his possession again?) let it pardon his (not sure why this is suum instead of illius - is it because habet stand aside as the true subject?) error. Thanks for anyone’s help 
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“est tamen et ratio, Caesar, quae moverit illum / erroremque suum quo tueatur, habet.”
“There is nevertheless also a reason, Caesar, that influences him and is able to make his own discrepancy thereby defensible [wherefore it/he would/could defend]”
habeo = to have the knowledge, means, ability, be able, know how to do or say anything (L&S)
Benigne praebes, Adriane! I was not aware of this use of “habeo”, so thank you so much for pointing this out. One query: “Suum” must be referring to Romulus, but “suus” and “se” also only refer to the subject of the clause, so must that not imply that habet is Romulus rather than the “ratio”? Therefore, technically, would it read something like: “There is nevertheless also a reason, Caesar, which influenced him, and (lit.) he had power over his error by which it might be pardoned”?
Yes, I put “it/he” because the subject could be Romulus but not 100% must suum refer to the subject of the sentence. It can refer to the subject of the discourse. (A&G §300, note.)
Ad clausulae principalis subjectum normaliter pertinet, nonnunquam subjectum orationis.
Brilliant, thanks .
Also, is it common to form perfect passive indicatives as perfect participle + fuit instead of p.p. +est? I found this today: numero turba notata fuit? Thanks.
I would say not. Maybe someone else knows better. I think: “The bunch that follows was one notated by number”, i.e., adjectival or substantive, not verbal.
Non est, dicam. Forsit alius melius monebit. Meâ sententiâ, adjectivi vel substantivi in isto loco non verbi est participium.
OK, great, thanks yet again Adrianus! 
Adrianus, in re “se” and it sometimes referring to the subject of the discourse, just a little later on Ovid goes on to say: nec toto perstare die sua iura putaris where obviously the sua cannot possibly refer to the verb introducing the indirect statement, putaris, but must, as you said, refer to an implied day subject!