NOT A TEACHER
3.This spring he started painting his cottage which has not been done for five years now.
(I believe that you need a comma after "cottage." "Which" here is being used to refer to the action of the cottage being painted because you said "done"; if you had said "painted," then I feel that "which" would refer to the cottage. In any case, I feel that that the clause is non-defining. I feel that it would defining only in something such as:This spring he started painting the cottage that had not been painted for five years.)
6.The president, who has always been healthy. has now began to show the first signs of illness. (I believe this is excellent, except for the little mistake. It should be begun.
Let's see what the teachers tell you and me. I am a learner just like you, and I think that you have done a great job.