Learn Italian: Lascio i biglietti sugli zaini. - I leave the tickets on the backpacks.

Sugli is the combination of the preposition su (on) and the definite article gli (the). We must use gli instead of i because the next word, zaini, is a masculine plural noun starting with the letter z.

What would the singular version of sugli zaini be?

It would be sullo zaino (on the backpack). Masculine words starting with z take the article lo in the singular (su + lo = sullo) and gli in the plural (su + gli = sugli).

There is no word for 'I' in this sentence. How do we know who is leaving the tickets?

The verb lascio ends in -o, which is the regular 'I' form for most Italian verbs in the present tense. Because the ending already tells us it means 'I leave', Italians usually drop the pronoun io (I).

How do the nouns biglietti and zaini change if I only have one ticket and one backpack?