Learn Hardcore Italian: Martedì mangio un panino in giardino. - On Tuesday I eat a sandwich in the garden.

In Italian, when you indicate a specific day as the time of an action, you normally don’t use a preposition. You simply say martedì + verb. If you wanted to express a habitual action (“every Tuesday”) you could say di martedì, but for a one-off or scheduled event you drop the preposition.

Why is io omitted before mangio? Shouldn’t it be Io mangio un panino?

Italian verbs are conjugated for person, so the ending -o in mangio already tells you it’s “I eat.” Including io would be redundant, unless you want to add emphasis (“Io mangio, non tu”).

Why is the present tense mangio used if this is talking about a future event?

Italian often uses the present tense with a clear time expression (like a day of the week) to talk about the near future. It’s perfectly natural to say martedì mangio… instead of the future martedì mangerò…. Both are correct, but the present is more colloquial for scheduled plans.

Why do we say un panino but no article before giardino (“in giardino”)?

Un panino needs the indefinite article un because you’re referring to “a sandwich.”
In giardino is an adverbial phrase of place that traditionally drops the article. In Italian certain locations used adverbially (like a casa, in città, in giardino) don’t take an article.

Why is the preposition in used with giardino instead of a or al?

Use in for spaces considered “areas” or enclosed places (e.g. in ufficio, in biblioteca, in giardino). If you wanted to express motion toward a specific garden (e.g. “I go to the school garden”), you’d say vado al giardino della scuola (where al = a + il).

Why is Martedì capitalized here? I thought Italian days are lowercase.

You’re right that within a sentence Italian days are normally lowercase. Here Martedì is capitalized only because it’s the first word of the sentence. Otherwise you’d write martedì.

Why does Martedì have an accent on the final i?

In Italian, words stressed on the last syllable often carry a grave accent to show where the stress falls. Martedì is pronounced mar-te-DÌ, so the accent marks the stressed ì.