This mobile phone is too expensive for my father to buy.

⇒ This mobile phone is so expensive that my father can't buy it (cấu trúc so ... that) 

 => This mobile phone isn’t cheap enough for my father to buy.

Cả hai cách đều đúng bn nhé.

Cấu trúc: S+tobe+too+adj+to.

=>C1:S+tobe+so+adj+that+mệnh đề.

=>C2:S+tobe(not)+adj+enough+for+...

Nhưng trong một số trường hợp,k thể dùng được cả hai cách mà chỉ dùng được C1(nghĩa là cách 1 thông dụng hơn)

This mobile phone is too expensive for my father to buy.

⇒ This mobile phone is so expensive that my father can't buy it.