Choose the correct form of the verb. This room ______ (not to use) by teachers.

The sentence is in the passive voice, indicating that the room is acted upon. The present simple passive is formed with 'is/are + past participle'. Since 'room' is singular, 'is' is used. The negation 'not' is placed after 'is'. Therefore, the correct form is 'isn't used'.

Answer: isn't used