Write down what the students were doing when the teacher came in. 1) Max/ stand/ at the door. 2) Anna and Alice/ discuss / the new film. 3) Mark / learn / a poem by heart. 4) Helen / read / a text book. 5) Two girls / write something / in their notebooks. 6) Oliver and Tim / make fun / of John. 7) Alex / look for / a pen / in his bag.

1) Max was standing at the door.
2) Anna and Alice were discussing the new film.
3) Mark was learning a poem by heart.
4) Helen was reading a text book.
5) Two girls were writing something in their notebooks.
6) Oliver and Tim were making fun of John.
7)Alex was looking for a pen in his bag.
В данном случае используется время Past continuous. Для его образования нужно поставить вспомогательный глагол to be в прошедшее время: was ( для I, she, he) и were ( для you, we, they) и после причастие (Paticiple I) смыслового глагола, которое образуется путём добавления окончания -ing к основе. В примерах two girls, Oliver and Tim, Anna and Alice эквивалентом является местоимение they ( поэтому используем were) , Алекс - , Max, Mark - he, поэтому используем was.