Learn Italian: Questo è il suo posto.

It wouldn't change at all! In Italian, the possessive adjective agrees with the object being owned, not the owner. Because posto (seat) is a masculine word, you must use the masculine il suo, whether the seat belongs to a man (his) or a woman (her).

Why do we need the word il before suo?

Unlike English, Italian almost always requires a definite article (like il, la, i, or le) right before a possessive adjective. You can think of it as literally saying 'the his seat'.

Could il suo posto mean anything else besides 'his seat' or 'her seat'?

Yes! If you are speaking to someone using the formal 'you' (Lei), the possessive is also suo (often capitalized as Suo). So in a formal setting, this exact sentence translates to 'This is your seat'.

Why do we use Questo instead of Questa for 'This'?

Questo ('this') is acting as a pronoun here, and it must match the gender of the noun it points to. Since posto is masculine, we use Questo. If you were pointing to a feminine noun, like a chair (sedia), you would say Questa è la sua sedia.