For the Italian:
Ha costruito un tempio per quando la pioggia sarebbe arrivata.
Your first guess is the simplest, and probably also the clearest translation of your Italian sentence into English:
- He built a temple for when the rain would come.
It translates pretty cleanly from the conditional in Italian because English shares with Romance the use of the conditional in these situations. Although many Romance languages additionally admit an imperfect subjunctive in “when” type clauses such as these — and if you dig back far enough you can find examples of English sometimes doing so as well — this was never done consistently at any point in our history, and present-day English certainly does not do so.
That’s why these don’t sound grammatical to our ear, or at best hopeless archaic and possibly even confusing:
- He built a temple for when the rain *were come.
- He built a temple for when the rain *were to come.
It might be possible to argue that should could work here, but I’m not going to try because it sounds too formal.
However, a simpler version is possible using the simple past:
- He built a temple for when the rain came.
That does lose a bit of the aspect, but that’s probably ok here. It’s also possible to swap around the come verb into an -ing progressive:
- He built a temple for the coming rain.