Translation:When the stone is thrown, it's difficult to get it back again.
"Smutt" comes to mind.
This is only true for definite nouns, like "barnet", and only in certain dialects (like the Oslo dialect they teach in this course). For past tense verbs, the t is pronounced. It's how you tell that a verb is in past tense.
I think it needs to be as shown, with the whole "Nå steinen er kastet" clause counted as the first element of the sentence, thus requiring the main verb in the sentence to come second (thereby pushing 'det' back one place).
On the audio, "kastet" ble uttalt mer som "kasta". Although I know that that is not the standard form of the past participle being taught in this course, I also know it to be a very commonly used form in actual speech. So I wrote the sentence with "kasta", and it was, of course, marked wrong. I reported the audio, but I do think that under the circumstances, both forms of the past participle should be accepted.