With an appropriate modal verb:

(1)Vicky's engagement ring is enormous! It ________ have cost a fortune.

(2)We are wondering how they ________ challenge that expert.

dynamic adjectives

tag question

extraposition

When the simple present refers to the present time,what are its characteristic uses?

restrictive adjectives

What's the function of relative pronoun?

Using a YES-NO question:

I don't know her telephone number.

Using an exclamation:

These old men are very nice people.

Using the passive voice:

We must put an end to this state of things as soon as possible.

Using subjunctive mood:

Why didn't you ask them to deliver your parcel?