10 While the teacher were speaking, the students were listening to him.

В данном предложении нарушено правило согласования подлежащего и сказуемого. Слово «teacher» стоит в единственном числе, поэтому глагол должен быть в форме «was».

Исправленный вариант:

While the teacher was speaking, the students were listening to him.

Ответ: While the teacher was speaking, the students were listening to him.