Learn Portuguese (Portugal): Parto cedo.

Parto comes from the infinitive verb partir (to leave). Even though it belongs to the -ir group, regular verbs ending in -ar, -er, and -ir all share the exact same ending when talking about yourself: you just drop the last two letters and add an -o to make the "I" form.

Where is the word for "I" in this sentence?

In Portuguese, you often drop the subject pronoun like eu (I) because the verb ending already shows who is doing the action. The -o at the end of parto makes it perfectly clear that "I" am the one leaving, so eu is not strictly needed.

I thought the verb partir meant "to break". Can it also mean "to leave"?

Yes, partir is a very common dual-purpose verb! It means "to break" in some contexts, but it also translates to "to leave" or "to depart" (especially when starting a journey). You just have to use the context to figure out which meaning fits best.

Does the word cedo change if the subject is feminine or plural?

No, cedo is an adverb (a word that describes an action) meaning "early". Unlike adjectives that change to match masculine or feminine nouns, adverbs never change their endings. It will always be cedo.