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`1` I have not written a letter for `5` years .

`-` S + have not/has not + V3/V_ed + O ... : Ai đó đã không làm gì trong bao lâu ...

`2` How long is it since you last built the house `?`

`-` When + did + S + V-inf + O ... `?`

`=>` How long is it since + S + last + V(quá khứ) + O ... `?`

`3` It's `3` days since I last met him .

`-` S + last + V(quá khứ) + time .

`=>` It is + time + since + S + last + V(quá khứ) + O ...

Các công thức đổi HTHT - QKĐ

Type 1: 

S + has/have + not + V3/ed + for/since + time 

--> S + last + V2/ed 

--> The last time + S+ V2/ed + was + time 

--> It is / has been + time + since + S+ (last) + V2/ed 

Type 2: 

How long + has/have + S+ V3/ed?

--> When + did + S + V1?

--> How long is it + since + S+ V2/ed ?

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1. I have not written a letter for 5 years.

2. How long have you built the house?

3. It's 3 days since I last met him.

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