He started to learn English in 1990 and in the years ________ he kept on using it in his work.
Do you know the police ______ searching for a thief in the woods now ?
When he was young, his parents died and he had to _____ himself.
Whether life will continue on the earth will depend on _______ this problem can be solved.
Only when it is hungry______ a human being.
In such a cold day, people prefer to stay at home _______ go out.