`=>` the last time they cleaned their room was two weeks ago 

`->` the last time `+` S `+` V2/-ed `+` was `+` khoảng thời gian `+` ago : lần cuối ai làm gì đó là từ khi nào

`=>` they last cleaned their room two weeks ago 

`->` last , ago `->` QKĐ 

`->` cấu trúc quá khứ đơn : `(+)` S `+` V2/-ed 

`-` It's two weeks since they cleaned their room

`=>` The last time they cleaned their room was two weeks ago

`=>` They last cleaned their room two weeks ago

`@@@@@@@@@@@@@@`

It's + khoảng thời gian + since + S + (last) + Ved/`C_2` + (O) + .....

`<=>` The last time + S + Ved/`C_2` + O + was + khoảng thời gian + ago

`<=>` S + last + Ved/`C_2` + O + khoảng thời gian + ago

`->` Lần cuối cùng ai làm gì đó.