It is no use crying over spilt milk. (we think…)

It is difficult for me to solve the problem myself. (I found…)

Where he has gone is not known.（it作形式主语）

To master two foreign languages is not an easy thing.（it作形式主语）

Learning a theory without practice will be useless.（it作形式主语）

“Will my parents scold me?” (He wondered)

“Who have you caught stealing her purse?” (I asked the police)

“How many guests were present at the party?” (I asked the organizer)

“Was it a good idea to play that trick?” (I wondered)

“When is the aircraft supposed to arrive?” (I asked them)