under + 数字（包括a 在内）构成的词组，under 是副词，不是介词。理由如下：

一、副词完全可以修饰数词，让“确数”变成“概数”。

我们以Twenty people were seriously injured in the traffic accident. 为例说明该问题。本句的主语是“二十个人”，是一个很准确的概念。如果记者写报道时对这个数字不很确定的话，他就可以在twenty people 前加一个表示“概数”的副词。例如：

Under twenty people were seriously injured in the traffic accident.

Nearly twenty people were seriously injured in the traffic accident.

Approximately twenty people were seriously injured in the traffic accident.

Roughly twenty people were seriously injured in the traffic accident.

Almost twenty people were seriously injured in the traffic accident.

About twenty people were seriously injured in the traffic accident.

Around twenty people were seriously injured in the traffic accident.

Over twenty people were seriously injured in the traffic accident.

Exactly twenty people were seriously injured in the traffic accident. （Exactly也是副词修饰twenty，只是强调这个数字的准确性而已）

二、副词under 与相似副词的类比:

含有-ly 的nearly, roughly, approximately, exactly 是副词，应该没有异议，这同时也说明副词修饰数词，是完全成立的。

以上例句，如果承认about，around 也是副词的话，那么，我们为何不承认under，over 也是副词呢？

三、介词短语作定语，只能后置不能前置。

Under twenty people were seriously injured in the traffic accident.

本句，如果认为under 是介词，那么，它的宾语是谁？是twenty? 还是twenty people?

如果介宾是twenty，那么介词短语Under twenty 作people 的前置定语？——不成立！

如果介宾是twenty people，那么介词短语Under twenty people 作句子主语？——更不成立！

你查查词典就知道了。under“少于，不到，在……以下”，看似副词，实为介词。例如：

Her weight fell to under seven stones. 她的体重减到了7英石以下。《柯林斯高阶英汉双解词典》

The journey has taken us under two hours. 这次旅行我们花了不到两个小时。《英语介词习语词典》

▲表“（数量）超过，多于，不止”时用over，这时的over也是介词。例如：

It costs over ten dollars. 它值十多美元。《现代英汉综合大词典》

Over a hundred people were present. 有一千多人出席。《英语介词习语词典》

此类问题我曾和刘老师讨论过，刘老师的说的很有道理，但很难找到依据，所以我们都无法说服对方。

http://ask.yygrammar.com/q-21982.html

just under a year ago

under prep.（年龄或数量）不足，低于，在…以下 (英文解释：If something or someone is under a particular age or amount, they are less than that age or amount.） 

These are jobs for those under 65.

这些是适合65岁以下人群从事的工作

Nearly half of mothers with children under five have a job.

孩子不满5岁的妈妈中近一半有工作。