16. We finished all our homework. We played some games of chess. (before)
We had finished all our homework before we played some games of chess.
17. Peter has lived in Viet Nam for only a month. He knows how to use chopsticks when eating. (although) 
Although Peter has lived in Viet Nam for only a month, he knows how to use chopsticks when eating.
18. It is cheap to go by train. It is expensive to go by air. (cheaper)
It is cheaper to go by train than go by air.