`15.` The last time he wrote to me was in April.

`16.` She last had a swim five years ago.

`17.` They haven't talked to each other for two months.

`18.` She hasn't seen her elder sister since 1999.

`19.` It's years since she wrote to me.

`20.` It's ten years since Robert and Catherine married.

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`o.` Thì hiện tại hoàn thành`:` S + have/ has + V(pp) + since + mốc thời gian.

`->` Thì quá khứ đơn`:` The last time + S + Ved/ V2 + to be + in + thời gian.

`o.` Thì hiện tại hoàn thành`:` S + have/ has + not + V(pp) + for + khoảng thời gian.

`->` Thì quá khứ đơn`:` S + last + Ved/ V2 + khoảng thời gian + ago.

`o.` S + have/ has + (not) + V(pp) + for + khoảng thời gian.

`->` It's + khoảng thời gian + since + QKĐ.