$III.$
$1.$ He often went to school by bike.

$⇒$ He used to go to school by bike.

$2.$ "Don't leave the door unlocked ", his mother said to him.

$⇒$ His mother told him not to leave the door unlocked.

$3.$ The front yard is to small to play soccer in.

$⇒$ The front yard isn't big enough to play soccer in.

$4.$ No city in Viet Nam is bigger than Ho Chi Minh city

$⇒$ Ho Chi Minh city is the biggest city in Viet Nam.

$5.$ Shall we go out tonight ?

$⇒$ Let's go out tonight.

III)
1 . He used to go to school by bike

2 . His mother told him not to leave the door unlocked

3 . The front yard isn't big enough to play soccer in

4 . HCM city is the biggest city in VN

5 . Let's go out tonight !