英语中含有副词 only 的句子算否定句吗？或者说是肯定形式否定意义？比如：

He is the only child in the family. 他是家里的独生子。

She is the only person who wants the job. 她是唯一想做这项工作的人。

我提出这个疑问是因为有的书上说：can 表示推测时通常用于否定句（包括含有only的句子）和疑问句。如：

You can only be caught if you and the police arrive there at the same time.

不能说含有only的句子就是否定句，也不能是是肯定形式表示否定意义。只能说含有only的句子有时候隐含着否定的意味。例如：

He is the only child in the family， isn't he?

The family has no other children, has it?

She is the only person who wants the job, ins't she?

No one else wants the job, don't they?

You can only be caught if you and the police arrive there at the same time.

You can't be caught unless you and the police arrived there at the same time.

但句子本身却是肯定句，反意问句要用否定形式。

only的语义与否定有关的另一个证据是only+状语前置，像否定意义的状语前置一样，也需要倒装。

Never does it snow in Hong Kong.

Only yesterday did I know the news. (=I didn't know the news until yesterday.)

关于only与否定有关，Huddleston著The Cambridge Gramma of the English Langrage, P. 96-97在讲述句首的only引起部分倒装的时候有如下论述：

Initial only

Only is not a marker of negation: He has only seen her once (unlike He hasn't seen her once) is a positive clause. But it has a close relation with negation, for such an example entails that he has not seen her more than once. And this connection with negation is reflected in the fact that as far as inversion is concerned it behaves just like a negative:

She had complained only once.

Only once had she complained.

As with negative, inversion does not apply unless only has scope over the clause: compare Only a few days later, he moved to Bonn, where only applies just to a few days within the initial AdvP.

这一部分内容我翻译如下：

句首的only

only不是否定标记词。He has only seen her once是一个肯定句。但它与否定有密切联系，因为这个例句意味着他见她的次数不超过一次 (He has not seen her more than once)。这种与否定的联系反映在这样一个事实上：在是否引起部分倒装的问题上，only的作用与一个否定词的作用完全相同：例如：

She had complained only once.

Only once had she complained.

和否定词的情况相同，除非only的修饰范围覆盖整个分句，否则不倒装。比较下句：

Only a few days later, he moved to Bonn. 这个句子中only只修饰句首副词短语中的a few days.

网友的这个句子中如果没有only，则不宜用can, 而应该用may.

You may be caught if you and the police arrive there at the same time.

这是说话人根据“你”可能和警察会同时来到这里而作的主观推测，这种肯定句中的主观推测是不能用can的。但句子一旦用了only，虽然表面上看仍是一个肯定句，但实际隐含了否定的内容，即：只要你不和警察同时到达，你就不会被抓。因此，only的使用，使can用在了形式上的肯定句中。这就是网友问题的本质。

结论：only不是否定词，含有only的句子也不是否定句。但only与否定有密切联系。正是这种与否定的联系，使only在用法上有与否定词类似的作用。only与can合用，使句子与否定有了联系。