Đáp án:

Giải thích các bước giải:

`17`. `text{Never before has she been so happy}`

Cấu trúc:

Never before + trợ động từ + S + V

`18`. `text{Only after the train had left did she realize that she hadn’t locked the door}`

Cấu trúc:

Only after + S + had + V3/ed + did + S + V

`19`. `text{Seldom do temperatures in Spain fall below 0°C}`

Đảo ngữ:

Seldom + do/does/did + S + V

temperatures là số nhiều --> dùng do

`20`. `text{On no account is the door to the basement to be left open}`

Đảo ngữ:

On no account + be + S + to be + V3/ed

`21`. `text{Only by learning hard can you pass the exam}`

Đảo ngữ: 

Only by + V-ing + can + S + V

`22`. `text{Only when I don’t have anything else to do do I watch television}`

_ Only when + mệnh đề = chỉ khi...
Đảo ngữ:

Only when + S + V + do/does/did + S + V

`23`. `text{Directly in front of them was a great castle}`

in front of + N

a + adj + N

`24`. `text{Bob didn’t remember his mother’s birthday, nor did he remember his sister’s}`

Cấu trúc thì quá khứ đơn:

S + V2/ed + O

S + did not + V + O

Did + S + V + O?

`25`. `text{Only by shouting and waving their arms did they manage to get our attention}`

Only by + V-ing = chỉ bằng cách...

Đảo ngữ:

Only by + V-ing + did + S + V

`26`. `text{Rarely does his teacher get so tired}`

Đảo ngữ:

Rarely + does + S + V + O

`27`. `text{Only if you speak French will you understand him}`

Đảo ngữ:

Only if + S + V + will + S + V

`28`. `text{Barely had I started speaking when he interrupted me}`

Barely ... when = vừa mới... thì...

Đảo ngữ:

Barely + had + S + V3/ed + when + S + V2/ed

`29`. `text{One of the best universities in the world is Harvard}`

Cấu trúc:

One of + the + adj-est / most adj + N + O

`30`. `text{Under the tree was one of the biggest lions I had ever seen}`

Đảo ngữ:

Place + be + S + O

Cấu trúc thì quá khứ dạng tobe:

S + was/were + adj + O

S + was/were not + adj + O

Was/Were + S + adj + O?

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