`1.` It's one year since Jane last wore that skirt.

`→` Phủ định của hiện tại hoàn thành : S + have/has not + VPII + for/since thời gian

`=`  It’s + time + since + S + last + V2/ed + O

`2.` It's a month since the students last went to school.

`→` Phủ định của hiện tại hoàn thành : S + have/has not + VPII + for/since thời gian

`=`  It’s + time + since + S + last + V2/ed + O

`3.` I have learned English since I was in grade `6.`

`→`S + started/began + VING/TO V + thời gian

`=` S + have/has + VPII + for/since + thời gian

1.Jane hasn’t worn that skirt for one year. It’s a year since Jane hasn’t worn that skirt

2. The students haven’t gone to school for a month. It’s a month since the students haven’t gone to school

3. I started learning English when I was in grade 6. I have started learning English since I was in grade 6

THÌ HIỆN TẠI HOÀN THÀNH:

• Khẳng định: S + have/ has + Past participle (V3) + O. 
• Phủ định: S + have/ has + NOT+ Past participle (V3) + O. 
• Nghi vấn: Have/ has +S+ Past participle (V3) + O? 
• for + khoảng thời gian (for a year, for a long time, …)
• since + mốc/điểm thời gian(since 1992, since June, …)