49. My father hasn't seen his brother for nearly 20 years.

 - for+khoảng thời gian (thì Hiện tại hoàn thành)

50. How long is it been since you learnt to drive?

 - since+ cột mốc thời gian: từ khi.... (sau since nếu là một mệnh đề thì ta chia thì quá khứ đơn)

51. I haven't gone swimming since we were in Spain.

52. It's weeks since I have tidied this room.

49. My father hasn't saw his brother for nearly 20 years.

→  It’s + time+ since + S + (last) + V2/ed: Đã được bao lâu kể từ khi ai đó làm gì

= S + have/ has + not + V3/ed + for + khoảng thời gian

50. How long is it since you learnt to drive?

→ When + did + S + (last) + V0 ?: Ai đó (lần cuối) làm gì từ khi nào ?

= How long is it + since + S + V2/ed ?

51. I haven't gone swimming since we were in Spain.

→ The last time + S + V2/ed+ was + time + ago: Lần cuối cúng làm gì từ khi nào

= S + have/ has + not + V3/ed + for + khoảng thời gian

                                                       since + mốc thời gian

52. It's weeks since you last tidies this room.

→ giải thích tương tự câu 49