how do i change these three sentences to active voice:

1) the two lessons were the only ones planned so far...
would it be: these two lessons are the only ones planned so far

2) although some found the lesson to be hard...
3) this was taught during the second lesson

Active voice means that the subject is the actor in the sentence. In sentence 1, there is no actor, although it is implied that the teacher is doing the planning. "The teacher plans only two lessons so far." Or, "The teacher has planned only two lessons so far." The teacher is the actor, plans the verb. Do you see how it goes?