4, The rope had been cut, so it was obvious that the lamb had been stolen.

It is obvious that…显而易见

5, Dimitri at once set out to find the thief. He knew it would not prove difficult in such a small village.

set out to do sth. 开始着手做某事

it would not prove difficult 是不难做到的

prove 是连系动词，作“证明是”讲，该句相当于it would not be difficult

6，He told him he had better return it or he would call the police.

had better...or...最好...否则...

e.g. You’d better be there or you will never see me again. 你最好到那去，不然你就再也见不到我了。

or: 否则

e.g. He can’t be ill or he wouldn’t have come. 他不会生病要不他就不会来了。

or else: (口语) 否则……新概念2composition

e.g. Your room is so dirty. Clear it promptly, or else you should move out. 你的屋子真脏。立马打扫干净，否则你就搬出去。