a. Who will help you?

Ans: By whom will you be helped?

b. Tell him to come.

Ans: Let him be told to come.

c. Enter the room by this door.

Ans: You are ordered to enter the room by this door.

d. He killed himself.

Ans: He was killed by himself.

e. He made arrangements to leave the house by him.

Ans: Arrangements were made to leave the house by him.

f. Promises should be kept.

Ans: One should keep one's promises.

g. English is spoken all over the world.

Ans: People speak English all over the world.