Поставьте глагол в нужной форме, применив правило согласования времен:
1)He said we (may) keep the books as long as we (to like).
2)We thought that he not (to be able) to make his work in time and therefore (to offer) to help her.
3)When I came they (to tell) me that he (to leave) half an hour before.
4)It was soon clear to the teacher that the control work (to be) a difficult one.

1)He said we might keep the books as long as we  liked.2)We thought that he wouldn't be able to make his work in time and therefore  offered to help her.3)When I came they told me that he had left half an hour before.4)It was soon clear to the teacher that the control work had been a difficult one.