“only﹢if条件句”放在句首，主句谓语用部分倒装。如：
Only if a teacher has given permission is a student allowed to enter this room.
only +状语成分，并放在句首，后面句子需要使用倒装结构
Only if a teacher has given permission is a student allowed to enter this room.
(1)only+介词短语
Only in this way are you able to do it well.
(2)only+时间副词
Only them did I realize my mistake.
“only＋if条件句”放在句首，主句谓语用部分倒装。
Only if a teacher has given permission is a student allowed to enter this room.
Only if you ask many different questions will you acquire all the informationyou need to know.
(3)“only＋when状语从句句”放在句首，主句谓语用部分倒装。
Only when he is seriously ill does he ever stay in bed
Only when you finish your work can you go home .
（备注1：放在句末一般不倒装）
I told him he would succeed only if he tried hard.
我告诉他，只有努力才能成功。
(备注2： 如果only后接的不是状语，则无需倒装。
Only members may use the bar.
主句可以倒装.但不绝对！
Only if 条件从句在前，中间无逗号时主句要倒装!
Only if you have interested in learning，you will learn well. 是对的！
You will learn well only if you have interested in learning. 是对的！
Only if you have interested in learning will you learn it well.主句倒装，也是对的！
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