Learn Hardcore Italian: Non lasciarlo sul tavolo, potrebbe cadere. - Don’t leave it on the table, it might fall.

In negative commands addressed to tu, Italian uses non + infinitive, and object pronouns attach to that infinitive: non lasciarlo. While you might sometimes hear non lo lasciare, the standard, most idiomatic form is to encliticize the pronoun. (For a formal “Lei” command you’d instead say non lo lasci.)

Why is the second clause in the conditional (potrebbe cadere) instead of the present tense (può cadere)?

The conditional mood (potrebbe) expresses a possibility or uncertainty—equivalent to English “might” or “could.” Saying potrebbe cadere (“it might fall”) is softer and more natural for a warning. Può cadere (“it can fall”) states a general ability rather than a likely outcome.

What does sul mean and why is it contracted?

Sul is simply su + il (“on + the”). In Italian, the preposition su contracts with the definite article il to form sul. So sul tavolo means on the table.

How would I change this to a formal command using Lei?

Use non + present subjunctive, and place the pronoun before the verb:
Non lo lasci sul tavolo, potrebbe cadere.

Is the comma before potrebbe mandatory?

No. Punctuation in Italian is fairly flexible. A comma is common to mark the pause, but you could also use a semicolon or start a new sentence. Its main role here is clarity.

How do I pronounce cadere, and where is the stress?

Cadere is pronounced [ka-ˈde-re], with the stress on the second syllable -de-.