Learn Hardcore Italian: Ho piegato il cavalletto e l’ho messo nel marsupio grande. - I folded the easel and put it in the large waist pouch.

Because piegare is a transitive verb (“to fold”). Transitive verbs in the passato prossimo take avere, not essere. You only use essere with certain intransitive verbs (often movement or reflexive verbs).

How does l’ho work here? What is it short for?

L’ho is the clitic pronoun lo (referring to il cavalletto) combined with the auxiliary ho. When lo comes before a vowel, it elides and becomes l’, so lo ho messo → l’ho messo.

Does the past participle messo agree with the pronoun l’?

With avere, the past participle normally stays in the masculine singular form. However, if a direct-object pronoun precedes a verb with avere, you must agree the participle. Here l’ = masculine singular “it,” so messo remains messo. If it had been feminine plural, you’d see messe.

What is nel? Why not in il marsupio?

Nel is the contraction of in + il. Italian often merges prepositions with definite articles:

di + il → del
a + il → al
in + il → nel

Why is the adjective grande placed after marsupio?

In Italian you can put many adjectives before or after the noun. Size adjectives like grande often follow the noun for a neutral description. Saying il grande marsupio would be fine, but it sounds a bit more emphatic or poetic.

What’s the difference between cavalletto and treppiede?

Cavalletto can mean an easel (for paintings) or a small stand/tripod.
Treppiede is the more technical term for a camera or surveyor’s tripod.

Any pronunciation tips for piegato and cavalletto?

Piegato: “pie” = [pyeh], then “ga-to.” The ie is a close-front diphthong.
Cavalletto: stress on the second-to-last syllable: ca-val-LET-to. Double tt means a slightly longer “t” sound.