`13`.It is sometimes hard to breathe in the city

`->` Thì HTĐ ( ĐT Tobe ) : S + is / am / are + adj / n ( `13 , 15` )

`->` `DHNB` : sometimes

`->` It is ( not ) + adj ( for O ) + to do sth : Như thế nào để làm gì

`14`.They have gone to work in larger towns

`->` Thì HTHT : S + have / has + VpII

`->` go to work ( v ) : đi làm

`->` SSH ( `14 , 15` ) :

`=>` S1 + is / am / are / ... + adj-er / more adj + than + S2 

`->` large `->` larger ( SSH ) `->` the largest ( SSN ) : rộng lớn

`15`.I think living in the countryside is better

`->` think sth + is adj ( -ing ) : Nghĩ như nào

`->` good `->` better ( SSH ) `->` the best ( SSN ) : tốt

`\text{# TF}`

13. It is sometimes hard to breathe in the city.

$-$ Có "sometimes" là dấu hiệu ⇒ dùng thì HTĐ.

14. They have gone to work in larger towns.

$-$ Thì HTHT: S + have/has + V3/VpII.

15. I think living in the countryside is better.

$-$ S + think + mệnh đề: ai đó suy nghĩ như thế nào.

$-$ Tính từ so sánh đặc biệt: good ⇒ better ⇒ best.

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$@haanx2$