Learn Italian: Ieri ho lavorato molto. - Yesterday I worked a lot.

We use the passato prossimo tense, which requires two words: a helping verb and a past participle. Here, the helping verb is ho (I have), and the past participle is lavorato (worked). For regular verbs ending in -are like lavorare, you drop the -are and add -ato.

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

In Italian, you usually leave out subject pronouns like io (I) because the verb form already tells us who is doing the action. The word ho specifically means "I have", so saying io ho lavorato is unnecessary unless you want to add special emphasis.

Could molto change its ending here, like molti or molta?

No. In this sentence, molto is an adverb meaning "a lot," and it is describing the verb (how much you worked). When molto describes a verb, it never changes its ending. It only changes its ending when it describes a noun, like saying "many books" or "a lot of pizza".

Does ho lavorato translate to "I worked" or "I have worked"?

It can be both! The Italian passato prossimo covers both the English simple past ("I worked") and the present perfect ("I have worked"). Because of the word ieri (yesterday), "I worked" is the most natural translation here.