第一幕的英语为 the first act。在这里act为名词，取“短节目”这一释义。

act：v.行动; 表现; 表演; 起作用;

n.行为; 行动; 法案，法令; 短节目;

第三人称单数：acts；复数：acts；现在进行时：acting；过去式：acted；过去分词：acted

相关例句：

1.第一幕，还有最重要的场景是由托尼·布莱尔，工党还有那群年轻的工党学生社团一块写的。

The first acts, the most important scenes were written by Tony Blair, New Labour and its youth wing -Labour Students.

2.对于电影剧本来说，第一幕无疑是其最为重要的组成部分。

The first act is the most important part of your screenplay.

3.房间的建立旨在描绘理查德·瓦格纳的《唐怀瑟》第一幕。

The room was built to illustrate the first act of Richard Wagner's "Tannhuser".