英语语法中的MOOD，通常译作“语气”或“式”，是一个语法范畴，它是区别说话人对动词所表示的动作或状态采取何种态度的动词形式。英语动词有三种语气或三种式，即陈述语气或陈述式(INDICATIVE MOOD)，祈使语气或祈使式(IMPERATIVE MOOD)以及虚拟语气或虚拟式(SUBJUNCTIVE MOOD)。

陈述语气或陈述式一般用来叙述事实或就事实提出询问，广泛应用于陈述句和疑问句，可以说百分之九十以上的英语句子用的都是动词陈述式；祈使语气或祈使式是说话人向对方提出要求，如下达命令、提出请求、劝告时所用的动词形式；比较麻烦的是表示说话人主观设想的动词形式，即所谓的“虚拟语气”或“虚拟式”。

根据传统语法，虚拟语气多至七种类型，即：

(1) be型

(2) were型(包括一般过去时等形式）

(3) had been型

(4) should be型

(5) should have been型

(6) should (would) be型

(7) should (would) have been型。

也有些传统语法学者将上述七种类型归并为四种，即

(1) be型

(2) were / had been型

(3) should be / should have been型

(4) should (would) be / should (would) have been型

这两种归类法(实际上是一回事) 长期以来在我国英语语法教学中被广泛采用。不足之处是，这两种归类法不分形式和内容，把凡能表达有关主观设想的语法手段统统划入“虚拟语气”。

在现代英语的动词形式中，“虚拟语气”的只有两种：一是 be-型虚拟式，二是 were-型虚拟式。

所谓be-型虚拟式是指不管主语是什么人称，动词律保持与原形同形，例如：

She insisted that I be / you be / he be sent to the front.

He suggested that I work / you work / he work with them.

It is important that he not leave / she not leave / I not leave / we not leave here.

所谓were-型虚拟式是指不管主语是什么人称，动词一律用 were, 例如：

They would wait if I were / you were / he were late.

We would not go if I were / you were / he were ill.

至于其它五种都不属于“虚拟式”, 例如：

[1] Far be it from me to hurt your feelings. (我极不愿伤你的感情。）

[2] If I were you, I shouldn’t do it. (假若我是你，我不做这事。)

[3] I wish I had known his name. (要是我当时知道他的名字就好了。）

[4] The doctor insists that he should give up smoking. (医生坚持他必须戒烟。）

[5] I was shocked that she shouldn’t have invited Phillis. (G122) (她居然不遨请菲利斯，我为之震惊。）

[6] I should / would be there by now if 1 hadn’t missed the bus. (要是我“当时”搭上了公共汽车，现在就到那儿了。）

[7] I should / would have been there by now if I hadn't missed the bus.(要是我“当时"搭上了公共汽车，早就到那儿了。）

按照七种类型处理法，上述七句都用了“虚拟语气”；按照两种类型处理法，只有[1][2]两句用了动词虚拟式，其它都不是，[3]为过去完成体用法，[4]-[7]为情态助动词的用法。

——以上内容主要参考章振邦主编的《新编英语语法(修订本)》第19章第1节。