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1. It's week since you last tidied this room.

→ S + have/ has + not + V3/ed  + for + khoảng thời gian: Ai đó không làm gì trong bao lâu

= It’s + time + since + S + last + V2/ed

2. We have looked for a flat for 2 months and we are still looking.

→ S + began / started + to V/ V-ing + time + ago : bắt đầu làm gì

= S + have/has + V3/ed+ for + khoảng thời gian

                                          since + mốc thời gian

3. I haven't been to a night club before.

→ This is the first time + S + have/has + V3/ed: Lần đầu làm gì

= S+ have/ has + not+ V3/ed+ before

4. This is the first time they have heard such a thing.

→ S + have/ has + never + V3/ed+ before: Ai đó chưa từng làm gì trước đây

= This is the first time + S + have/has + V3/ed:

5. This is the first time his authory has been challenged. 

→ No one has challenged his authory = His authory hasn't been challenged 

* bị động HTHT: S + have/has (not) + been + V3/ed + (by+O)