Learn Italian: Lascio il telefono sul letto. - I leave the phone on the bed.

In Italian, the preposition su (on) merges with the definite article (the word for "the") that comes directly after it. Because letto (bed) is masculine singular and starts with a normal consonant, it uses the article il. When you combine su and il, they merge to form the single word sul.

I learned that "partire" means to leave. Why use "lascio" here?

The verb lasciare (here as lascio, meaning I leave) is used when you leave an object behind or let go of something. You use partire when you mean "to leave" in the sense of departing or going away on a trip.

Why is there an "i" in "lascio"?

The dictionary form of the verb is lasciare. To make the "I" form, we drop the -are and add -o. However, to keep the soft "sh" sound of the sc, we must keep the i. If it were spelled lasco, it would be pronounced with a hard "sk" sound.

Why is there no word for "I" at the beginning of the sentence?

In Italian, subject pronouns like io (I) are usually left out. The ending of the verb tells you who is doing the action. The -o at the end of lascio already tells us that "I" am the one doing the action.