A cite to a traditional grammar, Allen & Greenough 517c:
c. Verbs and other expressions denoting necessity, propriety, possibility, duty, when used in the apodosis of a condition contrary to fact, may be put in the Imperfect or Perfect Indicative.
Such are oportet , decet , dēbeō , possum , necesse est , opus est , and the Second Periphrastic Conjugation:—2
“nōn potuit fierī sapiēns, nisi nātus esset ” (Fin. 2.103) , he could not have become a sage, if he had not been born.
“sī prīvātus esset hōc tempore, tamen is erat dēligendus ” (Manil. 50) , if he were at this time a private citizen, yet he ought to be appointed.
“quod esse caput dēbēbat, sī probārī posset ” (Fin. 4.23) , what ought to be the main point, if it could be proved.
“sī ita putāsset, certē optābilius Milōnī fuit ” (Mil. 31) , if he had thought so, surely it would have been preferable for Milo.
[*] Note.–In Present conditions the Imperfect Subjunctive ( oportēret , possem , etc.) is the rule, the Indicative being rare; in Past conditions both the Subjunctive (usually Pluperfect) and the Indicative (usually Perfect) are common.
http://www.perseus.tufts.edu/hopper/text?doc=AG+517&fromdoc=Perseus%3Atext%3A1999.04.0001
It’s not the most up-to-date treatment, of course, but its usefulness lies in its availability on-line, and it’s not as inaccessible, to me at least, as Pinkster.
Here’s a link to Woodcock, A New Latin Syntax (Cambridge MA 1958) at 156 (sec. 200), which is more recent than A&G:
https://babel.hathitrust.org/cgi/pt?id=uc1.32106010985098;view=1up;seq=184