1.

Is the common cold caused by a virus?

if

Could you tell me     the common cold?

2.

I don't think he saw the road sign.

have

He    the road sign.

3.

Summers in Brazil are hotter than in Puerto Rico.

hot

Summers in Puerto Rico    those in Brazil.

4.

Silver is half the price of gold here.

much

Gold costs    silver here.

5.

Mosquitoes spread malaria.

by

Malaria    mosquitoes.

6.

Morrison was unable to find anyone to help him.

succeed

Morrison    anyone to help him.

7.

Few people go to that island as it's so expensive.

cheaper

If that island    go there.

8.

There was great excitement at the end of the game.

very

People    at the end of the game.