Giải câu 4,5,6 thôi ạ

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

→ S1 + V + adj_er + than + S2

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

→ Although + S + V(chia theo thì), S + (be) + adj/adv...

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

→ It's isn't neccessary for + S + to + V `=` S + do/does + not + have to V_inf