一、为什么only在前是倒装？
你指的前倒和后倒是什么意思？ 相关语法规定如下： 如果only引导状语（或状语从句）放在句首时，句子（或主句）必须采用部分倒装语法，即把 句子（或主句）的 be动词/助动词/情态动词放到主语之前。
如果only引导的是状语从句，从句句序不变，主句部分倒装。 如果only引导的不是状语，则句子无须倒装。 如： Only in this way can you solve this problem. Only when it rains does he go to school by bus.
二、only引导的倒装句后面的时态看哪里？
要看句子里面的谓语动词，就能判断出它的时态。
三、only引导倒装句的归纳总结？
1、当“only+状语”位于句首时，其后习惯上要用部分倒装。其中，only后的状语可以是副词、介词短语、从句等。
Only in this way can we learn English. 只有这样才能学会英语。
Only then did I understand what she meant. 只有到那时我才明白她的意思。
Only by shouting was he able to make himself heard. 他只有叫喊才能让别人听到他。
注意，在only后作状语的是从句时，从句不要用倒装，要部分倒装的是主句。如：
Only when it rains do you feel cool. 只有下雨时才觉得凉爽一点。
Only when he returned home did he realize what had happened. 当他回到家里时，才知道出了什么事。
Only when we landed did we see how badly the plane had been damaged. 我们只是在着陆之后才看到飞机损坏的严重程度。
2、在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟的是主语，就不用倒装结构了。例如：
Only when the war was over in 1918 was he able to get back to work.
Only in this way can you learn English well.
Only after a bitter struggle was the aim achieved.
Only the teachers are allowed to use the room.(不倒装)
Only five men were hurt in the accident.(不倒装)
3、注意，only和所修饰的状语如果不在句首，则不倒装。
如：
The boy was helped out only when his father arrived in a hurry.
Only then did he realize the importance of English
只有在那时候他才意识到英语的重要性。
4、only then ,是过一小会儿的意思，一般用在句首，引导倒装句。
例句Only then did he realize the importance of English.
解析：在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟的是主语，就不用倒装结构了。
再比如
Only when the war was over in 1918 was he able to get back to work.(倒装句)
Only five men were hurt in the accident.(不倒装
四、only放在句首哪三种情况用倒装？
only放在句首用倒装的三种情况是：
1、only修饰副词放在句首时要用倒装。
Only then did he realized his fault.
只有到那时他才意识到自己的缺点。
2、only修饰介词短语放在句首时要用倒装。
Only in the way can you finish the task ahead of time.
只有用这种方法你们才能提前完成任务。
3、only修饰状语从句放在句首时要用倒装。
Only when the rain stopped could we go out to play.
只有当雨停了我们才能出去玩。
五、onlyif位于句首需要倒装吗？
only if位于句首，如果if引导的句子是状语从句，这种情况下需要倒装。例句：
1、Only if your mother has given permission are you allowed to go out at night
只有得到你妈妈的允许，你才可以在晚上出去。
2、Only if we understand can we work out that difficultproblem.
只有我们明白了，我们才能解出那个难题。
3、Only if the traffic light is red must the traffic stops.
只要交通灯变为红灯，交通就必须停止。