一般书上说到以否定词开头的句子都是说主谓宾结构，但如果主句是主系表结构呢？
今天看到一个句子觉得很奇怪：Never is he happy（他从不快乐）
还有：By no means is he the person we need（他绝对不是我们需要的人）
这里直接把整个系动词放到了主语前，主语后只剩下表语。为什么这么倒装呢？
当句子以具有否定意义的词开头时，
1. 在be的一般现在时am、is、are和一般过去时was、were作主系表结构句的系动词的情形，直接把be动词的相应形式搬到主语之前形成（事实上的完全）倒装句。例如：
He is never late for school.---> Never is he late for school.(他上学从不迟到。)
She was seldom happy at the time.---> Seldom was she happy at the time.(她那时很少开心过。)
2. 除了1中所述的两种情形，其它所有情形都是部分倒装，即把助动词或情态动词放到主语前面去。
①一般行为动词作谓语时
He never worked here.--->Never did he work here.（他从未在这儿工作过。）（改成倒装句时，由于增加了助动词did，原来的worked必须用原形动词work！）
We should never trust the enemy--->Never should we trust the enemy.（我们永远都不要相信敌人。）
John has hardly been trusted before.---> Hardly has John been trusted before.(以前约翰几乎没被信任过。)
If I had known that, I would never have told the truth.---> Never would I have told the truth if I had known that.(我要是早知道这样，我就永远不会说出真相！)
② 对于系动词be的非一般现在时和非一般过去时，也是要部分倒装的！例如：
I shall never be anyone else.---> Never shall I be anyone else.(我永远都不会成为其他任何人。)
They could never be in such a difficult condition.--->Never could they be in such a difficult condition.(他们永远都不会处于这么困难的环境里。)
③对于半情态动词have/has to与其它动词构成句子谓语的情形，若句子以具有否定意义的词开头，在主语前增加助动词do/does形成部分倒装。例如：
I seldom have to obey anyone's order.---> Seldom do I have to obey anyone's order.(我很少不得不听从别人的命令。)
John hardly has to work on weekends.---Hardly does John have to work on weekends.(约翰几乎不用被迫在周末工作。)
注：当have to前有其它助动词或情态动词时，当然要遵守一般部分倒装的规则，即把助动词或情态动词提前到主语前面：
I will never have to do that.--->Never will I have to do that.(我将永远都不需要那么做。)