谓语的数原则上与该结构中名词的数保持一致。
only的用法：
一、only引导的词组或句子放在句首作状语时，通常要用倒装结构。如
Only in this way can you solve the problem.
只有通过这种方式你才能解决问题。
Only if you study hard can you pass the exam.
只有努力学习才能通过考试。
Only then did I realize my mistake.
只有那时我才意识到错了。
二、not only...(but also)出现在句首时，如果架连接的是两个句子，常用倒装。如
Not only did we lose all our money,but we also came close to losing our lives.
我们不仅丢了钱，还差点把命丢了。
Not only did he speak correctly,but he also spoke easily.
他不仅说得准确，而且说得很轻松。
三、only放在
不定式前面，常表示不愉快或出乎人们的意料的结果。如
I went all the way to his home only to find hime out at a meeting.
我径直到他家里去，不料发现他出去开会了。
I hurried to the post office only to find it closed.
我匆忙赶到邮局，不料已经关门了。
She went home only to find her house burglarized.
她回到家里，发现房子被盗了。
四、“too...to...”结构中，表示否定意思，但在“only too...to...”结构中，only too = very，表示肯定意思。如
The patient is too weak to walk.
那个病人太虚弱了，走不动。
We are only too willing to do it for you.
我们非常愿意为你做那事。
I am only too delighted to accept your king invitation.
我非常高兴地接受你的邀请。
五、在“one of + 复数名词 +
定语从句”的结构中，定语从句一般被看作修饰复数名词，谓语要用复数形式。但在“the only
one of + 复数名词 + 定语从句”的结构中，定语从句却被看作修饰“the only one”，
谓语动词要用单数形式。如
This is one of the best novels that have been published in the past two years.
这是近两年出版的最好的小说之一。
He was the only one of the boys who was given a prize.
他是这些孩子中唯一受到奖励的。
She is the only one of the girls who has been to the US.
她是那些女孩中唯一去过美国的。
六、If only 为
虚拟语气的一种，表示愿望或一个未实现的条件，多用于
感叹句，其主句常常省略。如
If only she would marry me!
她要是能和我结婚该多好！
If only he had known about it!
他那时要是知道这件事该多好！
If only you could have been here earlier that day!
要是你那天再早点到这就好了。
If only there were forty-eighty hours in a day.
要是一天有48个小时就好了。