Io termino il lavoro alle cinque.

In Italian the verb ending usually tells you who is doing the action, so the pronoun is often dropped.
• Without io you can simply say Termino il lavoro alle cinque.
• You include io for emphasis, contrast (e.g. “Io termino alle cinque, tu finisci prima?”) or clarity in spoken language.

Termino is the first-person singular present indicative of terminare.
• It literally means “I finish.”
• Italians often use the present tense to describe scheduled or habitual actions, so it can also convey “I will finish (as planned) at five.”

In Italian most singular, countable nouns need a definite article.
• Il lavoro = “the work” or “the job.”
• Omitting il (“finisco lavoro”) sounds incorrect—always use il lavoro in this context.

Yes. The most common alternative is finire:
• Finisco il lavoro alle cinque.
Both verbs mean “to finish,” but finire is more frequent in everyday conversation.

Because cinque (hour) is feminine plural—le cinque.
• The preposition a (“at”) combines with le to form alle.
• So alle cinque = a + le cinque.

Absolutely.
• Terminerò il lavoro alle cinque means “I will finish the work at five.”
• Using present tense (termino) simply sounds more immediate or neutral for scheduled events.

• Sto terminando il lavoro means you are in the process of finishing right now.
• Termino il lavoro alle cinque indicates when the action will be completed (habitual or planned), not that it’s ongoing at this moment.

Yes. When you use terminare with an infinitive you add di:
• Termino di lavorare alle cinque.
• It’s equivalent in meaning: “I finish working at five.”

Italian typically requires the definite article with singular countable nouns.
• Omit il only in very abstract or idiomatic contexts, not here.

Time phrases are flexible:
• You can start with it: Alle cinque termino il lavoro.
• Or end with it: Termino il lavoro alle cinque.
Both are equally correct; choice depends on emphasis.