Reply
Tue 19 Apr, 2016 12:42 pm
I have been teaching them since 2015 but I (had already taught OR already taught) most of them during the previous two years.
I have been teaching them since 2015 but I had already taught most of them during the previous two years.
I have been teaching them since 2015, but I had already taught most of them the previous two years.
I would not have omitted the 'during'.