When the Principal entered the class, a student [tex]\sf\purple{was\:writing}[/tex] 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".
[tex]\huge{\textbf{\textsf{{\orange{My}}{\blue{st}}{\pink{iq}}{\purple{ue}}{\red{35}}{\green{♡}}}}}[/tex]
