请看下面的题目：

If he was here，he __________ us how to do it.

A. told                        B. would tell                        C. had told                        D. have told

此题应选 B。该题考查与现在事实相反的虚拟语气：此时条件从句的谓语动词用过去式，主句的谓语则要用 would (should，could，might)+动词原形：

If there were no subjective mood，English would be much easier. 要是没有虚拟语气，英语就会容易得多了。

If I knew his telephone number，I would ring him up. 我要是知道他的电话号码，我就给他打电话了。

在虚拟条件句，动词be的过去式通常用were(不管是单数还是复数)，但在现代英语，单数情况也可以用was：(www.yygrammar.com)

We could ask him if he were [was] here. 他要是在 这里，我们就可以问问他了。

但是在某些表达(If I were you，as it were等)中，be只用were这一形式：

If I were you，I should go and see the doctor at once. 假若我是你，我会马上去看医生。

The sky is covered，as it were，with a black curtain. 天空好像是被黑幕遮住了似的。

有时在虚拟条件句也可能用 could，might 等，那多半是因为语义的需要：

If he could help you，he would help you. 要是他能够帮助你的话，他会帮助你的。