Learn Hardcore Italian: L’insegnante indica l’errore con una penna rossa. - The teacher points out the mistake with a red pen.

In Italian the definite articles il and lo drop their final vowel before a word beginning with a vowel. The apostrophe shows this elision. So il insegnante becomes l’insegnante, and lo errore becomes l’errore.

How can I tell if l’insegnante is a male or female teacher?

Insegnante is an epicene noun (same form for both genders). You only know the gender from context or by adding a gendered adjective:

L’insegnante è stanco. (male)
L’insegnante è stanca. (female)
If you must specify you can also use il maestro/la maestra or il professore/la professoressa.

Why does rossa come after penna? In English we say “red pen.”

In Italian descriptive adjectives normally follow the noun: penna rossa. Placing rossa before the noun (rossa penna) is possible only for special emphasis or poetic effect and sounds unusual in everyday speech.

Why is the preposition con used here? Could I say something else?

To express the instrument used, Italian uses con + noun: con una penna rossa means “with a red pen.” You could also say usando una penna rossa (“using a red pen”), but con is far more common for instruments.

Why is l’errore (definite article) used instead of un errore?

Here the teacher points to a specific, known mistake (for example on the student’s paper), so the definite article l’ is used. Un errore would mean “some mistake,” indefinite and not referring to a particular error.

Why isn’t there an apostrophe in una penna? I thought vowels always elide.

Elision happens only when an article ending in a vowel precedes a word starting with a vowel. una ends in a, but penna starts with p (a consonant), so no elision: una penna, not un’penna.

What does the verb indicare mean here? Can I use another verb like mostrare or puntare?

Indicare means “to point out” or “to indicate,” often by gesture or marking.

Mostrare means “to show,” which is more general and doesn’t imply the physical act of pointing.
Puntare means “to aim” or “to point (a weapon),” and sounds too literal for marking homework errors.

If there are several mistakes, how would you say the sentence?

Pluralize both the article and the noun:
L’insegnante indica gli errori con una penna rossa.
(Still using one red pen.)
If multiple pens were used:
L’insegnante indica gli errori con penne rosse.

Could I say una penna di colore rosso instead of una penna rossa?

Yes, una penna di colore rosso is grammatically correct but wordier. Native speakers prefer the shorter una penna rossa.

Is penna the only word for “pen,” or are there other options?

Penna is the general term for “pen.” If you mean “pencil,” you use matita. For a fountain pen, say penna stilografica; for a ballpoint, penna a sfera or even biro.