`1.`The last time they had a vacation was in 2015.

`2.`The last time I saw him was 8 days ago.

`3.`I have not gone swimming since 2001.

`4.`We last saw Tom when he moved to London.

`=>`S + last + V2/ed + time

`=`The last time + S + V2/ed + was + time

`=`It's + time + since + S ( last ) + V2/ed

`=`Phủ định của hiện tại hoàn thành : S + have/has not + Vpp + since/for + time

1 - They haven't had a vacation since 2015

=> The last time they didn't have a vacation was 2015

S + have/has + not + Pll + since/for

= The last time + mệnh đề QK đơn ... + was...

2 - i haven't seen him for 8 days

=>The last time I saw him was 8 days ago

S + have/has + not + Pll + since/for

= The last time + mệnh đề QK đơn ... + was...

3 - i last went swimming in 2001

=> I haven't gone swimming since 2001

S+ last+ Vqkđ= S+ have/has + not + Pll

4 - We haven't seen Tom since he moved to London

=> We last saw Tom  whenhe moved to London

S+ last+ Vqkđ= S+ have/has + not + Pll