Learn Italian: Qualcuno ha rotto il termosifone in salotto. - Someone broke the radiator in the living room.

Qualcuno is an indefinite pronoun that is always grammatically singular. Even if you suspect multiple people broke the radiator, the grammar strictly requires the third-person singular verb ha. It also defaults to masculine, so any adjectives describing it would be masculine singular.

What is the infinitive of rotto?

It comes from rompere (to break). Rotto is a highly irregular past participle. Because rompere takes a direct object here (il termosifone), it uses the auxiliary verb avere.

Why do we use in salotto instead of combining the preposition and article to make nel salotto?

With rooms of a house, Italian prefers the simple preposition in without a definite article. You will see this pattern with in cucina, in bagno, and in camera. You only use the articulated preposition (like nel salotto) if you add a descriptive detail, such as nel salotto di Anna (in Anna's living room) or nel salotto nuovo (in the new living room).

Is termosifone the only word for a radiator in Italian?

No, you will also frequently hear calorifero used interchangeably for home heating radiators. Another word is radiatore, but that is generally reserved for the radiator inside a car rather than the one heating a house.