I just don't understand_____________ that prevents so many Americans from being as happy as one might expect.
It wasn’t until nearly a month later ____ I received the manager’s reply.
As is known to all language learners, the newly learnt words will soon be forgotten unless______ used in everyday communication.
The child asked his mother _______ go out to play tennis.
When changing lanes, a driver should use his turning signal to let other drivers know _______.
After dinner they talked about the persons and things _______ they still remembered.