I failed in the examination last term and only then ____ the importance of studies. 

So difficult　　　 it to live in an Englishspeaking country that I was determined to learn English.

“Well done, John! Not a single mistake____in your exam paper this time!”said the teacher．

So sudden ______ that the enemy had no time to escape.

In the dark forests ________，some large enough to hold several English towns.