譬如What if I came tomorrow instead of this afternoon ?
只有一个条件从句，那它的主句就一个what?还是省略了什么？
费致德《现代英语惯使用方法词典》对“What if...?”有以下讲解：
What if...?的意思是：
a.“假如......那就会......吗？”（What will happen if ...? What is the case if ...?）有时用虚拟语气，如：
Let people speak up, the sky wont fall and neither will you. What if you dont let people speak? Well then, you are pretty sure to tall one day. 叫人讲话，天不会塌下来，自己也不会垮台。不叫人讲话呢？那就难免有一天会垮台。
What if you go instead of me?（What would happen if you went to my place?）
b.假如......那又有哪些关系呢？（What does it matter if...?），如：
What if it is true?（What would happen, if it be true?）假如那是真的，又如何？
What if he does not come?他不来又有哪些关系？