4 I _______ the exercise _______ when the teacher came to my desk. (do)

В данном предложении необходимо использовать глагол do в форме Past Continuous, так как действие происходило в определенный момент в прошлом.

I was doing carefully the exercise when the teacher came to my desk.

Ответ: I was doing carefully the exercise when the teacher came to my desk.