Giải câu 4,5,6 thôi ạ

`4)` An underground train across London cost cheaper than a taxi.

`-` SSH: S + V + Adj/ Adv + er + than + N

`-` a + N

`5)` Although Luke wants to study overseas, he is afraid of living alone.

`-` Although + S + V, S + V: Mặc dù...

`-` be afraid of + V-ing: nỗi lo sợ `1` việc sắp xảy ra.

`6)` Cathy doesn't have to buy new textbooks because she can borrow them from the library.

`-` It isn’t necessary for sb to V-inf : cần thiết cho ai đó làm việc gì

`-`  S + don't/doesn’t have to + V_bare : không phải làm gì