WHILE THE TEACHER (EXPLAIN) WAS EXPLAINING THE SUM ON THE BLACKBOAR...

1. - DE KHAO SAT GVTIENG ANH TINH YEN BAI 2009
DE KHAO SAT GVTIENG ANH TINH YEN BAI 2009
1. While the teacher (explain) was explaining the sum on the blackboard, the children (throw)were throwing paper aeroplanes around the classroom.