1. 表示“在国外”、“到国外”等，是副词，不是名词，因此不能与 in, to, at 等介词连用；其前也不用冠词。如：

He has gone abroad with his wife. 他已与他妻子出国了。

His son was living abroad. 他儿子当时住在国外。

不能说：go to (the) abroad / live in (the) abroad

注意：abroad 之前虽不能用 in, to 之类的介词，但是却可以用介词 from。如：

He just came back from abroad. 他刚从国外回来。

2. 汉语中的“国内外”，说成英语是 at home and abroad。如：

He is famous at home and abroad. 他闻名国内外。

The situation both at home and abroad is in our favour. 国内外形势对我们都有利。

3. 由于是副词，不是形容词，所以不能用于名词前作定语。但是，若语义需要，它可置于名词后作定语。如：

My first journey abroad is something I shall never forget. 我第一次出国旅行的情景是我永远不会忘记的。

She used to be terribly shy, but a year abroad has completely transformed her. 她过去十分腼腆，但在国外呆了一年以后完全变了。

比较下面两句abroad与from abroad用作后置定语的区别：

Investments abroad set another record last year. 向国外的投资去年又创新经录。

Investments from abroad set another record last year. 来自国外的投资去年又创新经录。