Subj. Pluperfect in Lingua Latina

[The Sloth]

So far, I've been using common sense to make out the rules for tenses.. but, perhaps there are some idiomatic usages I'm not aware of.

For example, in Ch. 33:

"Iulius dubitabat num magister Marcum laudavisset."

That's fine: "Julius doubted whether the teacher (had) praised Marcus," meaning, praised Marcus prior to Julius's doubt.

But, in the Pensum:

"Si industrius fuisset, te laudavit nec epistulam scripsit."

"If you had been busy (working), he would have praised you and would not have written the letter."

Is that correct?

[adrianus]

Salve, Ille Segnis/The Sloth

Si industrius fuisset, te laudavit nec epistulam scripsit."

"If you had been busy (working), he would have praised you and would not have written the letter."

Is that correct?

No, it's not correct, The Sloth. Falsum est, Ille Segnis.
"Si industrius fuisses, te laudavisset nec epistulam scripsisset."

What you had ("Si" + pluperfect. subjunctive, + main clause in Perfect tense) is odd.

In Latin and English, there is no tense which anticipates (goes further back than) the pluperfect, so if the main clause is pluperfect ("he would have praised/written"—pluperfect subjunctive), here the "si" clause can go back only to the pluperfect also ("If he would have been busy" in Latin—conditional, it didn't actually happen). There is no "plu-pluperfect".
Et latinè et anglicè, tempus deest quod plusquamperfectum anteit. Si plusquamperfecto tempore est clausula major, sicut etiam hîc clausula conditionalis (cum "si"). "Plusquam-plusquamperfectum" deest.

[adrianus]

So far, I've been using common sense to make out the rules for tenses.. but, perhaps there are some idiomatic usages I'm not aware of.

Common sense gets you so far and no further. You have to learn the rules and their conditions. I'm like you, struggling to remember them.
Adeò nec plus ultrà, sensus communis te adjuvabit. Regulae et accidentia sua discenda sunt. Ego tui similis sum, et difficile mihi est ea memorare.

[The Sloth]

Ah, thank you. That makes things clearer.