17. He hasn’t had a swim for four years.

 -> He last swum four years ago

This last time he swum was 4 years ago

=> Tom hasn’t swum for 5 years

18.He started working as a teacher 6 months ago.

=> He has worked as a teacher for 6 months

19.It has been 3 years since we first went to Japan.

=> We have gone to Japan for 3 years

20.I last had my hair cut in June.
=> I had had my hải cut since june