Learn Italian: Quando ho saputo del problema, ho telefonato subito. - When I found out about the problem, I called right away.

In the past tense (passato prossimo), sapere changes meaning slightly from "knew" to "found out" or "learned" a new piece of information. If you wanted to say you already knew something over an ongoing period in the past, you would use the imperfect tense: sapevo.

Why is it del problema instead of using a specific word for "about"?

In Italian, to "know about" or "find out about" something is often expressed using the preposition di. Because the noun problema takes the masculine article il, the preposition di combines with il to create the joined word del.

Wait, doesn't problema end in "-a"? Why does it use a masculine article in del?

Yes, problema ends in "-a", but it is an exception to the usual rule and is actually a masculine noun. It comes from Greek, and many Italian words with Greek roots that end in "-ma" (like il sistema and il programma) are masculine.

Why is subito placed at the very end of the sentence?

In Italian, adverbs of time like subito (meaning right away or immediately) generally go directly after the verb they modify. Here, it naturally follows the completed past tense verb ho telefonato.