Learn Hardcore Italian: L'autobus è puntuale al mattino.

In Italian, when a singular noun starts with a vowel, you elide the definite article. Since autobus is masculine, lo becomes l' before the vowel a. You would never use la for autobus, because it’s masculine.

How can I tell that autobus is masculine?

The article l' comes from lo, the masculine singular form.
Loanwords ending in -bus (from English) are always treated as masculine in Italian.

Why is è (from essere) used here? Could I say arriva puntuale instead?

È puntuale means it is punctual, describing a general quality: the bus is always on time.
Arriva puntuale means it arrives punctually, focusing on the action of arriving on time. Both are correct but highlight slightly different aspects.

What does puntuale mean and how do I use it?

Puntuale is an adjective meaning on time or punctual.
You can use it:
• After essere: Il treno è puntuale.
• With verbs: Lui è sempre puntuale (He is always punctual).

What’s the difference between puntuale and in orario?

Puntuale emphasizes the trait of being on time.
In orario (lit. “in schedule”) focuses on fitting a timetable.
In transport contexts, they’re often interchangeable:
• L'autobus è puntuale.
• L'autobus è in orario.

Why is it al mattino instead of la mattina or di mattina?

Al mattino (lit. “at the morning”) is used for habitual actions within a schedule.
La mattina is more neutral: La mattina bevo caffè (In the morning I drink coffee).
Di mattina stresses “during mornings” in general.
Here, al mattino places the bus’s punctuality into the morning time slot of its timetable.

How would you say The buses are punctual in the morning?

Pluralize the article, noun, and verb:
Gli autobus sono puntuali al mattino.
Gli is the plural form of l' before vowels; sono is the plural of è.