`1.` I have never attended such an enjoyable wedding party before.

`->` This is the first time + S + have/has + P2

`=` S + have/has + never + P2 + such + a/an + adj + noun + before

`2.` Helen has worked here since `2000.`

`->` S + started + V-ing + in + time

`=` S + have/has + P2 + since + time

`3.` 

`-` Mark last went to the museum a year ago.

`-` Mark hasn't gone to the museum for a year.

`-` It's a year since Mark last went to the museum.

`->` S + last + Ved/V2 + time + ago

`=` S + have/has not + P2 + for + time

`=` The last time + S + Ved/V2 + was + time + ago

`=` It's + time + since + S + last + Ved/V2

`1` I haven't attended such an enjoyable wedding party before .

`-` This is the first time + S + have/has + V3/V_ed + O ... : Đây là lần đầu ai đó làm gì ...

`=>` S + have/has + V3/V_ed + O + before : Ai đó chưa bao giờ làm gì trước đó ...

`2` Helen has worked here since `2000` .

`-` S + have/has + V3/V_ed + O + time : Ai đó đã làm gi được bao lâu rồi ...

`3`

`@` Mike hasn't been to the museum for a year .

`@` Mike last went to the museum a year ago .

`@` It was a year since Mike last went to the museum .

`<=>` It + be + time + since + S + last + V(quá khứ) + O : Đã là bao lâu kể từ khi ai đó lần cuối làm gì ...
`<=>` S + have/has + not + V3/V_ed + O + time : Ai đó đã không làm gì trong bao lâu ...
`<=>` The last time + S + V(quá khứ) + O + was + time : Lần cuối ai đó làm gì là khi nào ...
`<=>` S + last + V(quá khứ) + O + time : Ai đó lần cuối làm gì khi nào ...