Learn Italian: Gli operai le assemblano attentamente. - The workers assemble them carefully.

To make it passive with venire (the current lesson's focus), the object ('them') becomes the subject. Since le refers to feminine plural objects, the passive sentence is: Vengono assemblate attentamente dagli operai (They are carefully assembled by the workers). Notice that the past participle assemblate ends in -e to agree with the new feminine plural subject.

Why is the word le placed before the verb, and what exactly does it mean here?

Le is a direct object pronoun meaning 'them'. In Italian, these pronouns usually go directly before the conjugated verb (le assemblano). Because it is le (feminine plural) and not li (masculine plural), we know the workers are assembling something grammatically feminine, like le macchine (the machines) or le parti (the parts).

Why does operai take the article gli instead of i?

Operaio (factory worker, laborer) is a masculine noun that starts with a vowel. For masculine plural nouns starting with a vowel, you must always use the definite article gli instead of i.

How is the adverb attentamente formed?