There used to have many more wild animals than today.
[have --- be ]注意：There used to be(过去有……),是“There be”句型.
Tom must pass the exam ,for there was a satisfied look on his face.
[pass --- have passed]因为根据后句的"there was"可知考试已经是过去的事了,所以用：must have done sth.(肯定已经做了某事),表示对过去的事做出判断.