`1`. have had

`-` HTHT, dh: since

`2`. have seen

`-` HTHT, dh: three times

`3`. left

`-` QKĐ, dh: ten minutes ago

`4`. have known

`-` HTHT, dh: for about five years now

`5`. didn't take

`-` QKĐ, dh: on our last holiday

`6`. How about notable sightings in the 21st century?

`-` How about + N/Ving?

`-` adj + N

`-` in the + thế kỉ + century

`-----`

`***` QKĐ:

`(+)` S + V2/ed

`(-)` S + didn't + V

`(?)` (Wh) + Did + S + V?

`***` HTHT:

`(+)` S + have/has + V3/ed

`(-)` S + have/has + not + V3/ed

`(?)` (Wh) + Have/Has + S + V3/ed?

Công thức thì HTHT

(+) S + has/have + V3/ed 

(-) S + has/have + not + V3/ed 

(?) Has/Have + S+ V3/ed?

DHNB: before, since + mốc thời gian, for + khoảng thời gian, yet, already, just,....

Công thức thì QKĐ (V: thường)

(+) S + V2/ed 

(-) S + didn't + V1 

(?) Did + S+ V1?

DHNB: ago, last (year, month ...), yesterday, in + năm QK, ...

---------------------------------------------

1. have had

HTHT + since + QKD

2. have seen

3. left

DHNB: ago (QKD)

4. have known 

DSHNB: for about five years (HTHT)

5. didn't take

DHNB: last holiday (QKD)

1. How about notable sightings in the 21st century?

`color{orange}{~LeAiFuRuiHuiBao~}`