句子副词（sentence adverbial）就是修饰整个句子的副词，而不是只修饰某一动词、形容词或另一副词等的副词。句子副词主要用于对整个句子或从句作出评论或解释。句子副词通常位于句首，有时也用于句末或句中。

下面一段说明摘自Collins COBUILD English Usage：

There are many adverbials which are used to show your attitude to what you are saying. These adverbials, which are sometimes called sentence adverbials, are explained below. Most of them are usually put first in a clause. They can also come at the end of a clause, or within a clause.

下面一段说明摘自Oxford Practice Grammar: Intermediate：

A sentence adverb is a word or phrase like certainly, perhaps, luckily, of course. It says something about the situation described in the sentence. The adverb can go in front, mid or end position. 

相关举例：

句子副词位于句首的例句：

Suddenly I heard a loud scream. 突然间，我听到了一声尖叫。

Outwardly she seemed no different. 表面上她看起来没有什么不同。

Truly, I don’t understand you sometimes. 的确，有时我不理解你。

Normally, I park behind the theatre. 通常我把车停在剧院后面。

Unwisely, she repeated the remark to her mother. 不明智的是，她把原话复述给母亲听。

句子副词位于句末的例句：

The region is important economically. 该地区的经济地位重要。

Anne is coming for a visit in March, supposedly. 据说安妮3月份要来探访。

I’ve known Barbara for years. Since we were babies, actually. 我认识芭芭拉很多年了，实际上我们从小就认识。

句子副词位于句中的例句：

The topic is undeniably an important one. 毫无疑问，这主题非常重要。

If they lose their hold, they will assuredly drown. 如果他们松手，肯定会淹死。

He was allegedly a leading participant in the coup attempt. 据说他是政变阴谋的主要参与者。