`2.` Understanding him is difficult.

`3.` Using this machine is easy.

`4.` Opening the window was very difficult.

`5.` Translating some words was impossible.

`6.` Maintaining a car is expensive.

`-----`

`-` It + tobe + adj + to Vo: Nó thể nào để làm gì

`->` Ving + O + tobe + adj: làm gì thế nào   

`2.` Understanding him is difficult.

`3.` Using this machine is easy.

`4.` Opening the window was very difficult.

`5.` Translating some words was impossible.

`6.` Maintaining a car is expensive.

`-` It + be + adj + (for sb) + to V: Nó là như thế nào (cho ai đó) để làm gì

`->` V-ing + be + adj + (for sb): Làm gì là như thế nào (cho ai đó)

`-` Thì HTĐ: `(+)` S + is/am/are + N/Adj

`-` Thì QKĐ: `(+)` S + was/were + N/Adj