Q.3 When the Principal entered the class, a student on the blackboard.​

When the Principal entered the class, a student on the blackboard.

Explanation:-

The given sentence is in past continuous tense.

The past continuous tense, also referred to as the past progressive tense, is used to express actions at a particular moment in the past. The action started before that moment but has not finished at that moment.

It is composed of two parts : the past tense of the verb "to be" (was/were), and the base of the main verb "+ing".

make sure the topic of your persuasive essay is not something someone else is writing about.