Learn Hardcore Italian: Io entro nella casa.

In Italian, nella is a contraction of in + la, meaning in the. You could literally translate it as "in the house." Writing in la casa would be grammatically incorrect; you need to merge in and la into a single word: nella.

Is it mandatory to use Io at the start of the sentence?

No, it's not mandatory. In Italian, subject pronouns (like io, meaning "I") are often omitted because the verb form indicates who is doing the action. You can say Entro nella casa and it would still mean "I enter the house."

What is the difference between entro and other forms of entrare?

Entro is the first-person singular present tense form of the verb entrare ("to enter"). Other forms include entri (you enter), entra (he/she/it enters), entriamo (we enter), entrate (you plural enter), and entrano (they enter).

Can casa mean both "house" and "home" in this context?